Stocks have value because they are cash flow generating assets, so if you sell low enough it will make economic sense for someone to buy, except for some liquidity catastrophe.

One thing though is that is unlikely that everyone would cash out below a certain price for a stock can be sold for parts. There would be at least some people realizing that they might as well hold, liquidate the company and make a profit out of it. So in practice there's a floor for the stock price (as long as assets > liabilities or similar).

That's not the case of BTC. Its value is completely depends on confidence. At no price can you be sure that you'll make a profit, you can only guess that it'll go back up, so there's no such floor.

There's substantial difference between a bank run and a Ponzi collapsing.

That's not really true, assuming it's a real company/bank with assets and revenue. If the stock is based on something with actual value there will many parties willing to buy the stock even if all of the current shareholders want to sell.

Behind a stock you've got a company with cash flow and assets. If nothing else you can sell off the office furniture. Crypto is backed by ... a prime number written on a piece of paper (minus the paper)?

Banks shouldn't ... in principle.