Compound interest is an exponential equation. Even 1% growth means doubling every 70 years and 1,000x growth every 700 years and 1 million x growth in 1400 years etc. Let's say US GDP is X$ and stocks are 'worth' a 100,000 X. At some point the market get's so far from the 'fundamentals' you get a crash, but it's more like a return to rational behavior.
PS: That's not to say tax breaks like 401k's cant shift things for decades. But, there is only so far you can inflate any bubble before it pops.
In terms of overall capital - money - this should be self evident. Money is a proxy for resources. Resources including labor are finite. There are only so many resources to chase. Even if the market extends control to all the world's resources there is a limit on its growth. Any numerical growth after that means nothing but inflation.
