The US could have had Southern Ontario in the Treaty of Paris. What if the US did get it?
Problem #1 - Securing the Upper Country/Pays de l'Haut (> Upper Canada > Canada West > Southern and coastal Northern Ontario) essentially requires the US also maintain control over the St. Lawrence River Valley (aka Canada proper at this time > Lower Canada > Canada East > Southern Quebec and the Côte-Nord). Which would need massive changes to the entire M.O. of the Continental Congress' plans for Canada. Chief among them would basically require not recognizing the Quebec Act as one of the Intolerable Acts (which would be very difficult, considering widespread anti-Catholic and Francophobic attitudes among Americans during this period, despite having France as a crucial ally and the usual exceptions), as well as developing a sensitivity to Canadien needs and aspirations as distinct from both the Anglophone minority and the French from France (if and when the latter eventually join up). Some of this sensitivity may be too radical for the Continental Congress to comprehend, as many of its delegates - particularly in the South - were content with trying to maintain as much of the pro-slavery status quo as possible without being subject to external authority. Since we are assuming here that the St. Lawrence River Valley remains separate from the US (which is a good call), that leads to:
Problem #2 - Massachusetts and New York. That portion of the Upper Country was subject to conflicting claims from both New York and Massachusetts, due to how the provincial charters defined the boundaries. Colonial boundaries during this period were very vague and often subject to a lot of inter-colonial border disputes. Usually these situations were either resolved through ad-hoc agreements or through warfare (with indigenous peoples caught in the crossfire). However, early America during the AoC period was far from united, despite all appearances, and in that context the border disputes would become a problem (particularly when it came to eventual Western settlement), since Canada's maximal boundaries out west are needed to keep the fur-based economy, and the networks with indigenous peoples the fur companies relied on, as a going concern, which is far less of a concern for Massachusetts and New York. It's pretty easy to wreck the US on the basis of that alone, but there would be more in play. Which leads to:
Problem #3 - Arrogance. Even after it clearly lost the hearts and minds of Canadiens, the Continental Congress/Army were insistent that Canada be added to the Union - which, as one could expect, led to that suggestion getting laughed out of the room in the eventual Treaty of Paris negotiations. I guess it's part of that, along with the weird contra dance among the various parties involved in the peace negotiations, that led to the OP's question. The main thing here is that both France and Britain were not keen on the US expanding that far north (as it would violate the Westphalian balance of power principle) and would prefer it remain dependent on at least one of those two for economic relations. The eventual settlement that occurred between the US and Britain, in fact, was way too generous to the US already, even as a compromise. To get to that maximum level of the Upper Country without the St. Lawrence River Valley would need
a different outcome for the Siege of Gibraltar, with Spain and France successful on that front. However, it would also probably need to have American arrogance tempered quite a bit, particularly since much of it was reserved for indigenous peoples. A watered-down version of the Proclamation of 1763 may have to be woven into the treaty to hold Americans to account on that (which, in light of everything else that followed IOTL, may probably help make life for Aboriginal peoples far less onerous
if settlement was more controlled). Leading to:
Problem #4 -
die Deutsche Sprache - or, IOW, the initial settlement pattern. Contrary to what is often assumed, not all early settlers in Upper Canada were actually UELs/Loyalists - in fact, most of them actually stayed in the US and adjusted to the new reality. Instead, a good portion of them were not so much Loyalists as Americans who, while not necessarily enthusiastic about how things were going, were more concerned about other things. In our terminology, we could probably call these people the "squishy middle", average people who didn't really take a side one way or another but who wanted to get away from the fighting. Also, for some reason, a lot of them happened to originate from Pennsylvania and similar areas (pre-1812 recruitment literature for colonizing Upper Canada referred to it as what America used to be before the Revolution interfered), hence why a significant portion of them were German speakers and/or of German ancestry (and why Canada holds some of the Western Hemisphere's oldest Amish and Mennonite communities, alongside the US). It was after 1812 and all that where the "Loyalist myth" and all that developed, and it worked. It's in that pre-Orange Order environment that Upper Canadian culture developed, and which Westminster and Whitehall sought to change to make it more pro-British than its residents would have cared - which, surprisingly, also worked. As a result, I would be a bit skeptical about massive changes in American settlement of Upper Canada as a result of the Revolution, at least up until 1812. I could a fair bit more of continuity than change, even if the wording is different. At the same time, I agree with
@Indicus that it might change the Maritimes and the St. Lawrence River Valley (should the US fail to acquire control of it) a fair bit in terms of the composition of the Anglophone community.
My guess is that Canada, as we know it, is aborted.
Not quite. There would still be a great deal of overlap between OTL and TTL east of the Ottawa River, even with increasing numbers of Anglophones (whether they be Loyalists, Nonconformists, Germans, or what have you). Where it would differ is that Britain could be far more generous to accommodating Canadiens' needs and aspirations than OTL, of which a taste for that was shown in the Quebec Act itself (which basically just legitimized what was already going on after the Conquest but with some deception to get around some of the strictures in place). The existence of people like the Canadiens and Britain's accommodation of communal needs IOTL already had a great deal of influence in extending toleration in Britain for religious minorities such as Catholics (much to the consternation of the C of E); since what was implemented in Canada IOTL had a tendency to show up elsewhere in Britain's overseas possessions in some form, it would be interesting to see how Britain would accommodate the Canadiens still further.
If Whitehall correctly diagnosed the problems that led to the Revolution in the first place (rather than what it would ultimately come up with IOTL), than the existing systems of government would be much different. Recall that up until 1791, Canada was governed under the Quebec Act, which promulgated a system similar to this diagram:
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File:Constitution-of-quebec-1775.png - Wikimedia Commons>
As a reward for perceived loyalty, the
Legislative Council could be expanded to include more popular participation from its residents. Now, it was recognized at the time that there were problems with the system as designed, because the British (incorrectly) believed that the remaining elites in Canadien society (the Church and the seigneurs) could will the habitants in line and pledge their loyalty to the King and the British Empire, but that was not how it happened, because most habitants were indifferent to whoever ruled them (not to mention the widespread distrust of the seigneurs and the hatred of having tithes reimposed on them). Other means of consultation and winning loyalty from the habitants would be needed, particularly once the professional classes start becoming a major force. Something would be needed to accommodate them within the existing system to promote further advancement (a corporatist solution, perhaps?). Furthermore, at this stage, Canadien nationalism was inclusive enough that anyone willing to learn French and become (or already is) a Catholic would be welcome into the fold; even if not becoming a Catholic, at least knowing how to speak the language of the majority of people and willing to learn their traditions, customs, and values would be appreciated. In that sense, the cosmopolitanism of the former French colonial regime was still in rude health. Because Britain was already moving towards a liberal direction anyway, albeit in fits and starts, this form of Canadien nationalism could be construed as being complementary with Britishness - if one knew how to harness it to make it work. Then it would be possible to bring Francophones (not just Canadiens, but even the Acadians in New Brunswick and in smaller communities elsewhere in the Maritimes and Newfoundland) on board with the project.
IMO, with Southern Ontario, the US is very likely to eventually control, at least, the Canadian West, as without Southern Ontario, Canada gets deprived of a lot of good land and transportation to the West gets more complicated.
Not necessarily - the St. Lawrence Valley was already densely populated as is; with the premature deprivation of the fur companies' territories and networks in the Great Lakes, they would need to find other sources of pelts that could be shipped for export - hence the HBC and Rupert's Land. Without the focus of Western settlement being a project of *Upper Canada, it would be further downstream in the St. Lawrence Valley where those population pressures would have to be addressed. So the *Canadian West ITTL would probably be more heavily Francophone than IOTL, which may or may not also help bring on industrialization (due to the Montreal-centric railway network eventually being adjacent to some of the mining regions) but would just basically turn it into a Brazil of the North without calling it such. Whatever *Canada loses to the US could be turned into an eventual advantage.
Therefore:
Is it possible that the Canadian colonies, seeing the impossibility of westwards expansion, would opt to join the US?
I don't think so. For the most part, the St. Lawrence Valley was perfectly fine as is without Westward expansion; if need be (as the ultramontane clerical-nationalists later on advocated) other marginal areas in *Quebec, even the north, could be brought under colonization. In addition, many Canadiens were already quite well aware of American anti-Catholic attitudes and all that entailed, including a sense their national institutions and
fueros (because using the analogous English terms "rights", "sovereignty", "autonomy", or "liberties" doesn't really capture the full dimension of what I think is under discussion), as well as continued usage of the French language, would not be respected. Instead, Quebec would focus inwards on building up a strong economic, political, cultural, and social base first; over time, as Western expansion occurs, the impossible would be made possible (even if only for political reasons regarding the HBC's continued hold of Rupert's Land and all that). By that point, where Western expansion becomes a possibility, Canada and the US would be too different to combine back together as it had been from 1763 to ~1781, and Canada (even in what IOTL would be seen as a truncated form) would want to keep it that way.