I wouldn't be surprised if the Bitcoin network already costs more than those mainframes in terms of carbon footprint - after all, the Bitcoin network is basically the world's largest supercomputer [0].
On top of that, you have to consider that those systems offer much more value than just a barebones ledger as the core of Bitcoin does. If you add in all those companies building stuff on top of Bitcoin, the comparison starts to become less favorable.
[0] Purely monetary terms are obviously different. The vendors of those systems inflate prices ridiculously because they can - banks have traditionally not been under much pressure to operate efficiently in this arena.
On top of that, you have to consider that those systems offer much more value than just a barebones ledger as the core of Bitcoin does. If you add in all those companies building stuff on top of Bitcoin, the comparison starts to become less favorable.
[0] Purely monetary terms are obviously different. The vendors of those systems inflate prices ridiculously because they can - banks have traditionally not been under much pressure to operate efficiently in this arena.