Earth's resources are strained, yes. However, taxing billionaires as you propose wouldn't necessarily pay for all the entitlement spending that the elderly require.

A solution to both of these problems is increasing total factor productivity (aka doing more with less). The big rise in living standards during the post-war boom was largely a result of a huge increase in total factor productivity.

This has petered out since the 1970s and a lot of the 'zero-sum' mentality/nostalgia for a 'Great America'/fighting over a fixed economic pie in the US today is because we haven't been increasing productivity (i.e. there hasn't been enough technological innovation and disruption) at a fast enough rate.