We're talking about two different issues here. First is the architecture/software of a mainframe system, with all of its redundancy and scaling. Second is the issue of hosting and maintaining your own hardware. On the first issue I'll admit I don't know enough about mainframes, perhaps they provide redundancy transparently to the programmer, but I'm a bit skeptical. Perhaps you simply think the redundancy is transparent because you understand the mainframe infrastructure deeply and internalized it? The second issue I'll push back on.

If you just want to host your servers on EC2, the SLA is 99.95% uptime. So that's about 4 hours of downtime due to outage a year. Put your servers in a multi-AZ autoscaling group and you're pretty solid. Bonus points for having autoscaling groups in more than one region. If you don't want to use any of the other services AWS provides, you can simply use them as a basic hosting service and get pretty amazing uptime. I've never used an old mainframe system, can you get greater uptime than that? Including hardware failures, power outages, network outages?