To be honest, if banks were the custodian of my cryptocurrencies, I wouldn't mind because the chances of my crypto being lost is going to go down drastically and if it was in an unlikely event got lost, the banks will have sort of insurance that will be in place to take care of the losses.
The risk of your crypto being lost is still the same if you are not careful. In fact, its higher since you may lose access to your funds anytime and someone else controls your coins. If you send your coins to the wrong address, your bank won't be able to reverse the transaction or do anything. You will still lose your coins. If someone has access to your bank accounts (steals your bank info), he will be able to steal all your coins and the banks can't do anything. And I doubt banks will have any sort of insurance to cover any loss you make while using crypto.