Centralized companies (i.e. Exchanges, investment firms or others) should have segregated accounts and must not commingle crypto assets
Crypto assets, like btc, eth, usdt, usdc can settle onchain 1-to-1 within a few minutes on a 24/7/365 schedule
Fiat accounts such as USD, Euro, Yuan have time-settlement constraints due to off-hours banking, international wire transfer delays, holidays and such
If I deposit $100K worth of BTC or USDT to Coinbase and want to trade for other crypto assets, there is an expectation that should I choose to withdraw at 11PM local time on Saturday that I will get the full amount minus network tx fees
Any crypto asset deposited on a centralized exchange should always have a 1-to-1 settlement to the blockchain
This is why a true crypto exchange charges exchange fees in cryptos to maintain all cryptos balances consistent 1-to-1 settlement to the blockchain
There is no expectation of the same settlement with fiat since not instant and has time-duration allowances, so the centralized exchanges can put in money market or buy treasuries to earn yields and stuff
One last note, if a big investor deposited $1B at a centralized entity and after a month of full fiat settlement and bought $1B worth of btc and decided to withdraw and was unable to do so because the btc's are not available, they were selling btc's that did not exist in their wallets and thus did not exist 1-to-1 on the blockchain
Similar to what happened with FTX but on a much bigger scale, a bank run of crypto assets should never be a problem for a crypto exchange and Binance has tweeted it will never be a problem there, we shall see. Coinbase has issued similar statement as well as Kraken