Because of short-selling, index funds could own all the market or more and active funds could still exist. For example we could imagine that index funds grow until they're 105% of the market, and active funds own 5% of the market long and 10% of it short. That adds up: 105% + 5% - 10% = 100%. And the active funds would still be doing all the price discovery. Of course the index funds wouldn't have all the assets-under-management, those would be split in the ratio 105:15 (with 10 of that 15 being collateral for the shorts).

In other words: it doesn't matter if index funds own all the shares, active funds can just make side bets with each other.