Putting all the eggs in the same basket to justify the basket. That's classical pointy-haired thinking.
It's been a long time now since I've managed hardware so I don't really know what the current situation is. But I happen to have ready access to a hardware document from around 2008 (the last time that I worked with such large system), and it lists a whole slew of RAS features that were copied from the mainframe. I expect that if I tracked down the corresponding document for a current high-end system that it would list even more.
That said, it was generally my experience that the whole platform family (low end to high end, new systems and old) was just rock-solid reliable - not at all like "The server crashed again - time to reboot it!" situation I usually found on the Wintel side of things. Plus stuff like malware was practically unheard of. And I've worked on systems that at any given moment might have thousands of users on them, and maybe tens of thousands to hundreds of thousands of jobs. From an operational perspective this might not have necessarily been the best idea, for a variety of reasons, but at least the system could handle it easily. And much like mainframes, unexpected outages were basically unheard of - if they ever happened they were extraordinary, jaw-dropping events. I'm not claiming the platform was/is perfect, though.