As long as the population is growing, it's a bearable burden on the young. But if you aren't replacing retiring workers with new ones, you can't keep funding the same quality of life in your society that you have grown accustomed to.
I don't know why Elon cares a lot about it. It's possible that he fears high tax rates more than most people. It's also possible he's completely out of touch with immediate problems.
https://interestingengineering.com/tesla-humanoid-robot-elon...
Robots work for a specific company, not society as a whole. Like software, they concentrate private wealth in the small number of elite who own them and actually make the social/Ponzi problem worse.
[1] - https://www.bbc.com/future/article/20220531-why-chinas-popul...
You cannot keep the actual rate of consumerism if population decline and corporations live in a fantasy world of unlimited growth, so they cannot keep growing if less and less people consume.