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.
And if you yourself became a co-owner of the bank and would store your crypto currencies and help others to do the same while earning without worrying about losses, you might like this idea.
https://bitcointalk.org/index.php?topic=5344349.0