Many banks, including very large ones, already have some amount of stuff running on public (& private) cloud infrastructure. Many other banks are preparing migrations. I, too, was surprised by this, considering only a few years ago they were very reluctant to even buy software from anyone who wasn't on a very short whitelist.

In the end, I guess it's all about the money ... and that includes the money it takes to find and train mainframe devs.

Source: spent a significant part of my career in fintech.