By symmetry, shouldn't the rapid growth of index funds imply the funds have paid inflated premiums to buy illiquid stocks? I suppose the 'bubble' claim is that they have, but that this is invisible because it has inflated the price of the underlying stocks as well so we still see the index funds priced the same as the underlying stocks.

At least for exchange-traded funds, it would seem that you don't have to actually destroy units of the ETF in the case of a sell-off. The ETF units would just sell at lower prices, just like when there is a 'sell off' of any stock - there are always equal numbers of buyers and sellers, you don't destroy units, you just move the price lower.

With index funds where you have an account directly with vanguard or whoever instead of buying units on an exchange, I'm not sure how it works in a sell-off. Perhaps they sell shares in the individual stocks, or perhaps they just try to sell off your shares bundled together by issuing more ETF units. I don't know what they do, but it seems like there are a bunch of options that should mean they don't have to sell off illiquid stocks on command.

I'm not sure. Happy to be enlightened. As much as I think about it, my intuition seems to consistently say that it's impossible for index funds to be broken in any meaningful way that's any different from the market itself or some sector thereof being in a bubble.