1) social systems can't handle it. You can't have a pension system where more retired people are drawing from it than young workers paying into it. It won't mathematically work.
2) a society with an aging population (where most of the people are old) is usually a society in decline. In a democratic society, it has the big problem where all the old people vote for their own interests, and don't know or don't care about young peoples' interests. So you get laws and policies which basically penalize not being old, plus you get old leadership that can't adapt to modern conditions.
Elon and Xitter suck, I agree, but that really has nothing to do with why an inverted population pyramid is so problematic.
As for Xitter, that (and his idiotic behavior there) wouldn't be a problem if everyone just stopped using it, and let it wither away and die.