"They won't be mainframes, because as the author does correctly point out, mainframes are unmatched at large-scale, high throughput, resilient, stateful, transaction-heavy workloads."

So like DynamoDB, Document DB, SQS, Service bus, and vast majority of managed cloud services?

Thats literally the entire reason to use cloud - to offload management of statefull services.

If I wanted just to run stateless docker containers, I can set that up myself on bare metal in a day.