If everyone runs for the exits then crypto suffers an old fashioned bank run, and it doesn't matter if it has any intrinsic value. A bank suffering a run still has loans on the books that are being paid back, but it has a cash crisis and people can't withdraw. A systemic failure starts knocking over institutions one by one as they each hit a liquidity crunch and don't have cash. Once the music stops playing everyone tries to grab some of the remaining chairs and get out. If all the cash is removed from the system and there's still billions in demand trying to exit while there's only a trickle of people trying to catch a falling knife, then it is going to overshoot and unwind to very nearly zero.
The reason why we have things like FDIC, is to prevent things like this, but crypto decided that was all old fashioned thinking. As a result, crypto is buttressed only by some billionaires who are true-believers in it, and if it only takes a few billion here or there then they may produce a bailout package for the system. If not, then the system will fail.
Any half decent exchange in a western country that holds them to account has over 100% of customer assets, they don't get special rules like banks who only need a few % of customer deposits.
Bank runs aren't really possible unless there is corporate fraud happening.
How do you propose to replace the $100bn online drug market?