The bank would have to either find a way to make significant profits from that heavy computation or reduce the computation; CPU time is an operational cost and banks exist to make profits. Crypto mining is designed to be inefficient; it's a security feature, not a bug.

And if the global banking system was pulling 2% of global power consumption the shareholders would be fetching the pitchforks and torches. I have no numbers off the cuff but I don't see a world where banks would be dumping crypto levels of money into power and server use.