New exchanges will emerge, plus there is Coinbase and Gemini and other regulated ones. You don't need to store your coins on the exchange if you're not selling them. If you want to buy, other people who want to sell will have their coins on the exchange, and after you buy you can withdraw them. I'm not sure why people conflate "liquidity" for trading with people just leaving them there when they're not selling them. Even when people
do leave them on the exchange, these coins are not contributing to trading liquidity because they are not up for sale.
The fact that crypto severely and relatively quickly punishes bad custodians who fractionally reserve and trade customer funds is a feature not a bug. And the reason this punishment happens quickly to crush bad actors is the "bank runs" necessary to trigger the event can occur within minutes of rumors or information being revealed to the market, and assets can be withdrawn globally very quickly. No other "banks" or exchanges or brokers are exposed to this much pressure punishing insider schemes like this.
Even the gold and silver markets have a lot of people claiming there is paper leverage way in excess of physical supply, and that large geopolitical interests suppress the price of gold so as to defend against it taking value from sovereign debt and currencies. I think there is some truth to this, but the reason it is allowed to persist and that there may not be true "price discovery" in gold and other commodity markets is that no one takes delivery, and so many simply leave their physical asset on custodian banks. Who knows what they are doing with this, or if their asset is truly allocated. It would require a crypto style "bank run" to punish these potential bad actors. Only crypto has this over and over again at scale.