If the bank adopts such a business technique of impacting the cryptocurrency space, it could help the market, but it's not advisable for banks to hold their customer's private keys; the security can be guaranteed since they save different assets for people too. If a hacker gets to their database, unless they save it in a paper, it'll be disastrous. Hence, I see positive reactions to the idea, many banking customers getting into crypto is a good wave, entering the market on the wrong side is a disadvantage to them.
I do not think the suggestion is about the bank having a hold of the private key in data form but rather in physical form where the owner of the private key will have their physical backup stored in a bank's vault. I also think that it is safe to keep our physical backup of keys in banks since banks had been proven to keep the physical asset in a safe condition and if something irregular happens, they have it covered.
This is the service I think banks can take advantage of Bitcoin enthusiasts and may be one of the sources of income of banks.