A significant number of people do not also control a significant portion of total stock ownership. Droves of retail investors could sell, but their ownership is rather inconsequential: the top 10% of Americans own 84% of stock. https://money.com/stock-ownership-10-percent-richest/
And personally I'd say that the 90th-99th percentile investor (if not 99.9th percentile) would still qualify as "retail" - what proportion of stocks do the top 1% (or 0.1%) of holders own?
If a significant number of people start moving money out of index funds, causing a decrease in the price of index funds, then that is just an opportunity for profit-making from other investors, which will stabilize the price.
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.
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.
I am not a financial planner, but just a few thoughts. First, folks who try to invest in crashes or divest on peaks tend to do it too early. Most of such events give you more than one opportunity to buy/sell and not being first in/out can give big advantages.
Second, we did not (yet?) see a big drop. A rise of 20% would put S&P at or near all-time highs. For comparison, during the 2008 crash, S&P dropped by over 50%, so the rise back would give you ~120% return. Thus the reward for "catching the knife" today is much, much smaller than in 2008.
Finally, a lot of stocks are now indexed and held in retirement accounts. Many folks there hold more than they should in stocks because they have been going up and seem to be the only game in town beating inflation. And all advisors say "leave your retirement accounts as is, long term view, yada yada". If a significant number of people start moving retirement money out of index funds, which they might do in uncertainty, there may be a lot of pain for stocks. For example, many smaller stocks just do not have the trading volumes to withstand even a moderate sales increase driven by index funds. My 2c.