Of course everyone cashing out at the same time would be akin to a bank-run. In practice this doesn't happen since people bought in at different times and have a different cost basis.
If I bought 10k of ethereum at one point in time and sold if for 100k, I put 10k into a pot which I took 100k out of.
Basic math tells me there won't be a "rising tide" for the boats that are left when anyone else pulls out.
I do not see how crypto is not a zero sum game, actually its a negative sum game if we deduct the mining costs, lost coins and the exchanges fees. Wasnt this supposed to work without whaleish, centralised exchanges?