Mainframe tech is specifically designed for handling these problems. Using “everything is an object” and “share nothing” tech, it should be possible to run these systems for decades without an outage. Using big-ass custom cloud platforms is a quick way to end up screwed.
This is why companies like Google, AWS and Microsoft are dumping IaaS and containers in favor or more reliable FaaS architectures. Yes, there were problems from the 60s to the 90s, but mainframes have generally always been far more reliable that the crap most vendors are passing as PaaS today.
Acid compliant share nothing record storage systems as well as FaaS as well as well designed load balancers and non-SAN object storage can offer almost zero-downtime (way better than five nines) platforms.
A vendor like this has absolutely no possible excuse for service outages. If they can’t do it themselves, they should call IBM and get it done right. It’ll cost a lot, but nowhere near as much as losing business due to letting IT people be involved in information systems.