If an index fund was an "atomic" independently traded unit, probably. But in index fund, for each $1 sold, managers must sell the underlying stocks that comprise the index in the exactly prescribed proportions. While this is not a problem for many heavily traded stocks, a lot of smaller stocks, even in SP500, do not have the volume to support a lot of shares changing hands, so their prices will likely collapse way more than the index. Which may trigger more selling. There was a discussion on this on HN a few months ago.

This is not a problem for actively managed funds, which have caveats that allow managers a lot of freedom. If there are no buyers for stock X, they do not have to sell it. Index funds do not get any such flexibility. The impact is amplified as the amounts invested in indices ballooned in recent years.

I do not think anyone knows if this scenario will happen and, if so, how bad the fallout will be, but there are systemic risks that go beyond sales uniformly depressing a price for a basket of stocks.