There seem to be studies after studies [0] of how wasteful Bitcoin (PoW) is - correctly so.

Are there any similar studies regarding the banking system?

With buildings and buildings full of offices just for banks, their IT infrastructure, plus ATMs, POS, etc.

How frequently are these systems replaced/upgraded?

How much energy are they using?

I think it would be interesting to make a comparison.

[0] (I have no sources, just based on seeing links to similar studies recently)