How is loosing territories some 5.000-10.000 km away an "existential threat"? Even if they wanted to, Japan had no means of successfully invading main land US.

The US justifies dropping the Nukes with it preventing an extraordinary loss of life if they had to stage an amphibious invasion of main land Japan. But at least the US could stage much closer to Japan, than Japan could to the US.

In the same wake the Britains loosing their empire was not an existential threat to the US just as much as the genocide against China was not an existential threat for the US just as the Holocaust and the genocides in eastern Europe weren't an existential threat to the US.

Okay good point about the threat WWII Japan posed but you're going too far in the other direction. When asked about going to war with America admiral Yamamoto said what amounts to "this is a bad idea don't". Now part of their failure definitely lies in poor leadership, and I can see the idea that if they'd concentrated on America instead of opening a war on three (two and a half?) fronts they might've made significant gains until America's industry caught up, but they simply didn't have the industrial base to keep the US down. The US also has very good natural defenses that you forgot to take into account. Remember: Their ships and airplanes were all handmade.