Banking is extremely variable. Instant transactions are periodic, I don’t know any bank that runs them globally on one machine to compensate for time zones. Batches happen at a fixed time, are idle most of the day. Sure you can pay MIPS out of the ass, but you’re much more cost effective paying more for peak and idling the rets of the day.
My experience are banks (including UK) that are modernizing, and cloud for most apps brings brutal savings if done right, or moderate savings if getting better HA/RTO.
Of course if you migrate to the cloud because the cto said so, and you lift and shift your 64 core monstrosity that does 3M operations a day, you’re going to 3nd up more expensive. But that should have been a lambda function that would cost 5 bucks a day tops. That however requires effort, which most people avoid and complain later.