Quality hardware has existed for years. At a ford motor plant they were doing an inventory and couldn't locate a 10 ton mainframe. It was working so well for 15 or so years the tribal knowledge of where it was physically located was lost.
Though it was likely easier to find than that Novell Netware server that was sealed behind some drywall, with only a stray network cable leaving any clue as to where it was.
I once only half jokingly suggested finding a missing data closet in a two million square foot distribution center by pinging a known IP from three or four aggregator switches across the building and triangulating the location on a floor plan. Sadly the people crawling around the ceiling found it before I could put my idea into practice.
Good job they found it!
Turns out a door was closed and a new one built to a hallway to another hallway and not properly labeled on the updated drawings. Had one of the boxes running a conveyor belt not have died, we'd never have looked.
Also, that doesn't cover other problems mentioned here, like natural disasters, ISP problems, etc.
Also cost does come in play as well. Multiple physical links in would be very expensive for what sounds like internal services. Likewise a natural disaster might cause bigger issues to the company than those internal services going down. They might still have offsite back ups (I'd hope they would!) so at least they can recover the services but the cost of having a live redundancy system off site might not justify those risk factors.
The customers requires are definitely unreasonable though. I'd hope those systems are regularly patched, in which case when is downtime for that scheduled and why is that acceptable but not when you're physically moving the server? I doesn't really make much sense; but then "not making much sense" also quite a common problem when providing IT services for others.
In general, we don't know much about this case. It's a post on Reddit, might not even be true. As is, it doesn't make much sense, but we don't know all the details, so maybe we jumped to conclusions.
Mainframe is not just a server. You can hot plug RAM on these things.