he has many ideas and projects but the overarching thing seems to be his 'preservation of consciousness' thing which means a permanent mars city, because he doesn't trust that a moon base wouldn't be attacked in wwiii.  his philosophy seems to hinge on the kind of idea that as the population grows exponentially in the future, the best benefit one can do for humanity is to plan for things that will make impacts felt in that future, not the now. if earth has like 100 billion inhabitants they'll look back at our 8 billion world and think we had it easy, etc.  personally i think it's fuckin stupid to assume we will have infinite population growth forever when basically every example we can see of countries developing curbs their growth. elon should not panic at shrinking populations if he actually cares about the future world's resources because we at least can help ward off things like crazy, world-war causing shortages of fresh water and arable farmland if there aren't nearly as many mouths to feed.
Japan