As long as companies can make it into the index, passive investors will funnel money into buying stock of these companies, no matter how badly these companies are run.
The trillions that mechanically and automatically flowed into index funds in pensions and 401k accounts must mechanically and automatically flow right back out after retirement, right?
Especially when younger generations are too poor to save for retirement and most companies don't offer pensions to younger workers any more, where will the inflows come from to offset the outflows?
Also, a significant part of the stock market is driven by foreign investment. The US has few capital controls and is an easy market for foreigners to invest in. Around 1/3rd of US stocks are owned by foreigners.
Even if the older generation sells during retirement, foreign investment will be more than enough to replace it.