and there is no malware for it.
Untrue, there were a few OS/2 viruses out there. I've personally run into samples of the MyName and Jiskefet viruses a couple of times back in the day. Not saying that it was common as it certainly wasn't, ranks up there with Linux malware, but there's no such thing as a bulletproof OS that's immune to malware.
Simply put, why change a running system for the sake of change?
Because things stagnate when they don't/can't evolve. What happens when something goes wrong, you need to add new systems but can't buy new licenses, you want to add new things into the system but can't, etc etc..? You get locked into something and you can't move on. Very bad from a business or technology standpoint.