> ...doesn’t that depend on infinite growth based ultimately on mostly finite natural resources, manpower, and land?

There is no practical limit to the price of the index. Shares may represent some claim on those things you mentioned, but shares outstanding have been steadily shrinking for decades, essentially since the dot-com boom [0]. Meanwhile, governments have been printing money like it's going out of style, and most of it has been ending up with the wealthy, who, lacking any practical way to spend it all on consumption, use it to bid up asset prices. It is no wonder that the price of a shrinking pool of actual resources [1] has risen dramatically when measured in a growing supply of money (which is only a relative resource, not a real one).

One could certainly look at the rise of the 401k as a retirement vehicle, and conclude that at some point those flows are going to reverse. One could also look at the US SSA statistics on the number of workers per retiree, and its trend over time, and paint a pretty grim picture, but is that enough to offset the combined forces of inflation and the compounding wealth of the ultra-wealthy?

Man, I don't know, either.

The good news is "nothing" is all you need to know to invest in an index fund.

[0] This has actually reversed recently with the AI boom, but it is the first time in a while that companies have tried harder to raise capital to grow the pie than to buy back shares to return cash to shareholders.

[1] Rather, a shrinking pool of claims on those resources, each claim increasing its value measured in real resources as companies buy back shares, an effect Buffett famously loves.