It's a huge supply/demand problem. At what price will someone forego Apple shares? What happens when Apple is 10%, 15% etc of the S&P 500 index? Where will these shares to sell come from? At this point, why would anyone holding Apple shares outright sell?
This demand may only lead to a self reinforcing feedback loop where: a greater market cap (3T?) -> higher index weight (10+%) -> greater buying pressure -> more shares locked up in index funds (not available for sale) -> repeat
This is explains why there has been increasingly volatile movements in Apple shares. This lack of share liquidity works both directions: buying and selling. Not enough active investors are available to step in when passive investors (who now make up an enormous portion of capital markets) decide to start selling index tracking funds in bulk.