You misunderstand.

You could run one of these things on a mainframe, because it's a zero-downtime machine - you can swap out parts of them as they run. But fundamentally it's a singleton. It is deeply naive to believe you can trivially translate that to something running on Kubernetes just "because Rust".

Of course most companies that need distribution do manage to do that, and eventually the banks will get there too. But it isn't feasible to do that by translating their existing non-distributed COBOL code, they need a fundamental re-architecture, and that is much harder.