[pedantic hat on] Bitcoin doesn't use encryption.

You mean digital signatures - and yes, we use signatures everywhere in public key cryptography.

not really. banking systems have firewalls and access controls. quantum computations would be useless.

Those don't really mean anything when an attacker can eavesdrop on customer and employee comms and possibly redirect transactions (MITM).

Banking communications and transactions will all be protected by quantum-resistant protocols and ciphers well before that will become a problem. Most of these already exist, and some of them can even be deployed.