Tip of the iceberg https://bitinfocharts.com/bitcoin/wallet/Bitfinex-coldwallet
Also, you're going with the assumption they shall be able to redeem 100%. Crash happens, like we have seen, but everyone cashing out their USDT is not a scenario going to happen. Or if you want to account for this scenario, then you can as well assume that crypto is going to disappear, and that would not happen without a cataclysmic event in the stock market either. Probably we will be back to the stone age at this point, and will have other things to worry about