If the trend continues of index fund growth as a percentage of the market, then the remaining active investors will gain an outsized influence on market direction. This is because index fund managers make very few trades on their own initiative, relying on market stats (trade history) to direct their own trades. Those market stats are built on the recent trades made by active investors.

If (when!) there is a correction, there will be analysis done on the cause (pick one or more of: student loan debt, sub-prime auto loans, trade war, something else) and then new investment strategies will be devised. And maybe passive investing will be shown to leave you vulnerable to a sea-change.