Agreed. I'd like to add that even if Swift/Visa/etc. combined use power plants worth of energy (which I actually assume they do, though it's hard to tell), they do orders of magnitude more payments with it.

Mainframes running a ton of financial transactions bill by CPU usage from IBM. Banks etc want to use as little computer resources as possible.

Running fiat involves more than just electricity.

People, offices and everything related requires power and physical location too.

>Why should Fiat money use more energy than global bitcoin adaption?

The problem was the original poster in this thread setting up the strawman that bitcoin is for every single transaction (or even a decent fraction of them).

But to answer your direct question: because the security and processing of it's transactions depends on the security of a nation state. That nation state's security stems from the military among other things. The energy expenditure is enormous and the influence of the nation state's desires on what transactions are processed is an everyday factor. This is obvious in watching the Justice Dept. interact with the major credit card companies.

Transactions that indirectly depend on bitcoin also increase its price, and cause more energy to be burned on mining. If bitcoin starts to secure any notable fraction of business it will cost much more than the amount of army needed to keep a currency stable.