Capitalism is no more a Ponzi scheme than, say, the agricultural or agrarian economy that preceded the Industrial-Revolution and Capitalism. The farmer needed to raise enough children to support him and his wife into old age. What is apparently problematic is modernity which liberates the individual from tribe, caste, clan and even from sex roles and marriage; but at the steep price of eventually being non-sustainable. Russia has more of a population regeneration deficit after decades of Communism. Western European nations also have population regeneration deficits after decades of Socialism, although less so than Russia. The United States population is at least regenerating itself even if immigration gives it a boost. What apparently attracts intact Hispanic and Asian families to emigrate to the U.S. is capitalism. The reason China and India are growing economically is the rise of what Marxists would call the bourgeoise family.
Read Chapter 7 “Family” in The Character of Nations: How Politics Makes and Breaks Prosperity, Family and Civility by Angelo Codevilla (revised edition 2009).
If you read the literature out there the people who harp on capitalism's need for expanding markets are leftists, socialists, communists, and environmentalists. To all of them it is a fatal flaw which results in the boom/bust cycles we appear to see hit us on a regular basis.
My own feeling is that boom/bust cycles are more a result of poor human judgment allowed to run amok due to government protection of crony capitalists. Get rid of the government protection and the capitalists would have to succeed in a real market which would tend to discourage speculative booms and busts.
However, I also believe that capitalism does not need expanding markets. I seem to be in the minority. Most pro-capitalist commentators use the term "expanding markets" to describe one of the benefits of capitalism, i.e. capitalists are better business people whose businesses grow faster than state-run enterprises so they are better able to expand their markets.
Your comment that modernity "liberates the individual from tribe, etc." is right out of the Communist Manifesto. Marx actually liked capitalism for its ability to separate people from their traditional affiliations. This left them free to focus on what he considered to be their main role in the world as economic pawns that need to gain control of the means of production to avoid exploitation.
Marx may have been right on some of this stuff, but in the end he posits an impossible utopian view that has led to the deaths of more people than all other philosophies and theologies combined.
If people freely choose to have fewer kids and/or freely choose to live simpler lives then they should do so.
I believe that the capitalist system is the system which will most easily adapt to such a change.
I do not believe that we need to get people to have more kids and buy more crap just so that we can feed Marx's caricature of capitalism.