I was reading an article earlier today that said passive investing is more than 50% of the market--and since most ETFs allocate by market cap, it causes a reinforcing feedback loop for market cap leaders.
For example, if they are only two companies, say with 1T and 4T market cap. If one invests 5M into a total market ETF, 1M is allocated to company A and 4M to company B. But since company B is 4x bigger than company A, the upward price pressure is the same for both companies.
In a hypothetical market with 100% ETFs, you’d have a status quo.
Edit: maybe not, since you have ETFs that invest in, say, Nasdaq only, which is tech oriented and would influence S&P500.
If you ever played Risk, or most other games, once the snowball starts, it's hard to stop it.
Of course, since the market has never been like this before, it's a speculation...