If everyone is old, who takes care of the old people? And don't say immigrants, because eventually every country will become old since birth rates are dropping everywhere, including Africa

Great until there's not enough carers to go around because of the shrinking population in the 20-35 demographic.

The government is made up of people. Working age people. The same people that fewer and fewer are existing.

At population level scales, "the rich" do not have the unlimited money pot that people think they do. The US spends 4.5 trillion dollars just on health care each year. If you completely liquidated all the wealth of the top 1% (ignoring that the fact that this is functionally impossible without massively decreasing its value; a stock is much less valuable if you know you can't hold onto it to collect future returns because the government is going to seize it), then the top 1% could fund American health expenses for ten years at the absolute most. Admittedly, the US is extraordinarily inefficient with health care spending, but if you adjust per capita spending to the levels of France, you still completely run out of money in 15-20 years.

This is a bit of an exaggeration, since you're obviously talking about a more limited problem then all medical expenses, but because the vast majority of medical expenses are incurred by the elderly, it's not as inapplicable as it might seem. Ultimately, funding sources need to be sustainable and not self-depleting, and for population-level spending, you pretty much always need to expand your funding beyond the ultra-rich. There's a reason why the excellent social services in Europe also come with a much higher tax burden for all people, not just the wealthy.