The very real probability of no longer having the strongest military in the world, which is something you express that you are still in favor of.
We already tried the strategy of waiting to intervene until Europe had already been violently consolidated into one enormous and war-obsessed power. Even if you don't care about the lives of people who live outside of the US at all (which, to be clear, I do care about), that mistake cost us alone over 400k lives to fix. And do you really think that if we'd just left Hitler alone he would have been too intimidated to attack us after he'd finished mopping up Europe?