Broadly, I've been doing a 5-fund portfolio with a mix of several index funds in decreasing percentages (total US stock, total international, US bonds, internat

Broadly, I've been doing a 5-fund portfolio with a mix of several index funds in decreasing percentages (total US stock, total international, US bonds, international bonds, REIT). If I had enough money for it to be meaningful, I'd probably invest something on the order of 0.1% in cryptocurrency too. The rationale is that uncorrelated assets result in average returns (across the set of assets) but decreased risk. What's unclear to me is the performance of e.g. the S&P 500 once the percentage of investments that are just index funds passes a certain threshold. Presumably if all investors were index fund investors the market would be super stupid, so is there a point before 100% where there will be significantly aberrant behavior? If so, what is that percentage and what is the behavior?