Depriving a belligerent nation of its ability to fight, with little loss of life or resources, is an acceptable outcome. Especially for those poor slobs who have to fight and die.

Foretelling the future is not an especially risk-free enterprise. Had the Japanese military survived on widely scattered islands for a few years or for decades, they would not have been a threat to the USA or the Allies. It would be like inmates running the prison. Guess what: They're still in prison. And if they get to be a problem then you crush them.

In any case, it wouldn't have taken more than 4 years to defeat them with little or no loss of life on the Ally's side. Remember, in 1945 we had the Bomb. In the 1950's we demonstrated you can sink an island with a bomb.