Market Cap != Liquidity
70% of the crypto volume is in USDT.
From 2020 (should still hold): https://coingape.com/tether-dominates-exchange-trading-with-...
If Tether collapses, so does USDT. If exchanges have majority of their volume in USDT pairs, then the market liquidity dries up and the whole thing collapse. Remember that those coins need to be cashed out for actual USD. But if that USD does not actually exist, what will you sell into?