> what does mainframe add other than complexity?

Compared to what? A cluster of x86 computers where all the security, cryptography, observability, reliability, availability has to be written by the owner on top of something like Kubernetes? And that can achieve the kind of throughput a mainframe has?

I'd say a mainframe is much simpler than that. It's already done. All you need to do is to sign the check and read the (hundreds of) manuals.