What happens to the market in 50+ years time if birthrate continue to decline?

As we all know the goal of long term investing is to pick stocks/funds that have strong potential and/or good track records, then watch them grow. But what causes that growth? More money coming in that money going out. This has been the case for many decades now, as our population has continued to grow, meaning more and more investors.

So what happens when boomers, that hold more wealth than every other generation combined, sell off stock en masse to fund their retirements? Is this something the stock market can ever recover from, given most of us in gen z cannot afford a family, so as we age outflow from the market will continue to exceed inflow.