Nope. If prices of the underlying stock collapse because of index selling due to movement out of retirement funds, then other investors will buy the underlying

Nope. If prices of the underlying stock collapse because of index selling due to movement out of retirement funds, then other investors will buy the underlying stock, stabilizing the price - even if they are not buying the fund. Selling doesn't automatically trigger more selling, that's just a media narrative. This might be true for very illiquid assets, but that is not publicly traded stocks.