The "long run" will be 5-10 years, not 150, because the other major powers won't stand for Japan locking them out of the China trade (on a cynical view) and won't stand for the abuses Japan would inflict in trying to hold China (on a more principled view).
Unlike Britain or Russia, Japan in the mid 20th century cannot sustain it's economy and industry without resources controlled by the very powers hostile to their Chinese ambitions.
I mean, this is the logic that led to OTL, after all. Assume a US government isolationist enough to let Japan defeat all open resistance in China - there's nothing to stop the next administration from cutting off the scrap iron, oil etc and pressuring the UK and Netherlands to do the same.
Again, presuming something close to OTL WWII is going on, they won't have much choice.
U.S. strategic planning in 1940-1941 envisioned bringing the Japanese on board as part of a global containment network against the Russo-German Axis. As late as November of 1940, President Roosevelt had advised Pacific commanders that the United States would not go to war over a Japanese invasion of European colonies according to the Joint Committee on the Investigation of the Pearl Harbor attack. IOTL Roosevelt was greatly relieved that Japan had taken the step of triggering the conflict with Pearl Harbor, and that “In spite of the disaster at Pearl Harbor and the blitz warfare with the Japanese during the first few weeks, it completely solidified the American people and made the war upon Japan inevitable” (
Roosevelt and Hopkins by Robert E. Sherwood, pg 335-336). Polling certainly backs this up, in that in February of 1941, Gallup conducted a poll in which respondents were asked “Do you think the United States should risk war with Japan, if necessary, in order to keep Japan from taking the Dutch East Indies and Singapore?”. The response was Yes (39%), No (46%) and No opinion (15%). By November of that same here, as a result of increased Japanese aggression and increasing global tension, that the response had flipped; asked "Should the United States take steps now to prevent Japan from becoming more powerful, even if this means risking a war with Japan?”, the response was now Yes (64%), No (25%) and No opinion (11%).
In a situation where Japan has refrained from expanding the war into Southeast Asia and the Nazis are taking full focus, Japan will take the backburner, so to speak, and by the time the Allies can return their focus (if?), the IJA will be able to present a fait accompli which the exhausted Allies will be in no position to contest. Should they try, Japan now has access to the resource base of China itself to make up for such; IOTL they had large industrial projects ongoing in Manchuria for oil, synthetic rubber, iron/steel, etc which were derailed by the path to the war. As late as 1940 they were still able to sign deals with American oil companies for equipment and help prospecting, for example.
Except Ichi Go is premised on going up against a China whose supply lines have been cut. Here, that's not the case. So long as China is receiving any appreciable amount of supplies through Burma, Indochina and the Soviets, Japan cannot knock them out of the war.
Not necessarily, no; none of these supply routes have sufficient capacity to solve the food issue of several hundred million people, which was one of the major effects of the offensive as it overrun much of the remaining good agricultural land. If WWII still happens as per OTL, the Soviet and French routes are taken out, while the British were successfully pressured to close the Burma Road in 1939.