Even if people were to do active bets on equity, what matters is the amount of money flowing in the asset class, so as long as there's an infinite stream of long investments into equity (due to 401k, etc.), the prices will rise.
You'd need people to actively balance their allocation between asset classes rather than stock X vs Y to counter those equity bubbles, but I don't think it's happening (and equity becomes too big to fail given the link with things like pension in the US).