The legacy system has one digital aspect of it, which is the SQL databases running on centralized servers. That may be the only energy efficient system. But compared to Bitcoin where transactions are processed by the nodes, which is literally the clients own devices already here the legacy system loses the energy war.

Then add to the legacy system bank office buildings with lot's of employees who's only job is to surveil and correct stuff that goes wrong because of bugs in the system. This is the alternative to mining, central verification by humans. Heating and cooling of large buildings allover the world is a massive energy consumer, add to that the fact that these offices needs prime real estate in bigger cities where energy is scarce and typically not from cheap clean sources like dams where most of the power for Bitcoin mining comes from.

Cash is part of the legacy financial system, it's not "paper money" as some muppets seems to think, it's literally just worthless paper. A fronted for the banks digital systems with no validation or verification of any kind. Easy to counterfeit, vulnerable to inflation. Requires heavy armored trucks to move around and large vaults to store as people would steal it otherwise. ATM's to withdraw and so on.

Then in the end, despite being digital it's still too slow, you can't wait days in the grocery store to make a payment, and the grocery store certainly wouldn't trust you to actually have the money you say you have. This means invoicing where some third party needs to store a shitload of personal information about you just so that debt collectors can find you if you try to evade paying. Bitcoin only takes 10 minutes to verify, on chain transactions. Most other cryptocurrencies are faster, then there's off chain payment solutions already which is instant and trust less which allows you to pay anonymously.

The legacy system uses third party card providers, who also needs to know a shitload of personal information about you, this is so that they can vouch for you in the store, despite not knowing for sure if you actually have the money. Because of that everyone has to pay insane fees, which covers their insurance against scammers and cheaters.

And despite credit cards, they too are way to slow which is why practically all public transit refuse to accept it, buses trains, metro, rental scooters anything with wheels you share has their own payment system where you top up in advance. Cinemas, the tivoli, museums, the circus and many other places uses tickets you buy in advance. These systems are often powered by diesel generators.

And people think it's a conspiracy that banks are looking to replace their crappy back end with blockchain technology before going cashless, a move that would make perfect sense, and the only threat about that really is the fact that banks suddenly gets cheaper and more convenient to use so that stupid people keep using them.