I think there was a period where everyone involved in the field knew the basics of WW2, the American Revolution and the American Civil War, but other historical periods weren't so well known. (I once spent some time in 2016 (or thereabouts) trying to explain the 'Civil War' to an American editor and he was rather confused about Cromwell and King Charles - what happened to Lincoln and Davis and why did a stand-in for General Lee take his army to London?)). The writer needed to explain more as he moved away from the well-trod areas; for example, if your POD involves British India, you have to tell the reader that Britain was ruling India at the time.
(Believe it or not, it took me a while to realise the reason British troops were in India during WW2 was that India was a British possession at the time - in my defense, i was 7.)
There's also a point where a change in the well-trod fields have an impact in the modern world - for example, the life pof everyone today would be different if the CSA survived the war or Hitler invaded the UK - when a change outside the well-trod would seem less impactful. The change may seem unimportant - what does it matter if the opium war doesn't take place - or too far in the past to produce a recognisable world.
A lot of that has changed, of course. New research, new voices, people writing from outside the US/UK/EUR: some things are no longer considered possible (Sealion, for example, or the Nazi Bomb) and others somewhat more plausible than we thought. Also more political issues - i knew a german who shied away from writing german-based timelines for obvious reasons.
I don't think the genre is stagnant. I think there are better things ahead.
