The point isn't that they were relevant - it's that every direct interaction with actual Japanese who were not prevaricating diplomats playing for time indicated that they had no inclination to surrender. Even the Germans "surrendered" only when Berlin was overrun. The Japanese were known for fighting to the last man and, in the post-war era, for holding out for decades past the war's end. The idea that the Japanese would put up less of a fight than the Germans short of some massive shock, might or might not be ludicrous, but it certainly ran against the experience of every encounter Allied forces had with them. Eisenhower's comment about the Japanese inclination to surrender sounded like his way of passing the buck - since as President Truman had taken ownership, why let any of the dirt get splashed on him?
“The Japanese were known for fighting to the last man and, in the post-war era, for holding out for decades past the war’s end.”
Nonsense. They resisted capture through suicide rather than permit allied capture. That’s not exactly ‘fighting to the last man’, is it?
The Allies took advantage of this by directing their attacks at strategic centres and cutting off areas that were well defending but unimportant to the general progress of the war. Japanese armies in northern Burma could do what? Attack china and be annihilated? March down the river to Mandalay and be annhilated? Do a forced retreat to where? Vietnam? Stay where they were without adequate supply? There were simply no good options for them.
That the Burmese armies were rendered insignificant is significant in that they could no longer hinder the progress of the war. What does it matter to the allies if you’ve got 20k Japanese soldiers cut off from civilized society in the jungles of Burma? What exactly are they going to do? What they ended up doing is the only sane outcome - marching to Mandalay and surrendering to the allies (instead of to the Chinese) who would have executed them. So that is what they actually ended up doing once they realized that there was no other option.
Arguing “Japanese had forces elsewhere that would have had to be engaged” Isn’t really a significant argument. By 1945, the allied blockade had forced these armies to hunker down into irrelevance.