And those OSs will run all the software I need? Mission-critical software that the existence of my company hinges on?
Before you retort with the obvious, i.e. why I allowed myself to be locked into the dependence on an OS that I have no control over, tell me what the alternatives were. I could have started out on a free OS. That would have entailed, though, that I'd have had to hire a programmer that can develop for this other OS, which would have been quite a bit more expensive some 10 years ago when cross platform libraries (like, say, Qt) were virtually unheard of. And more expensive was out of the question, because competitors did hire people who developed for WinXP, and my choice back then was only to either put my money on WinXP as well or fold right then and there because I could not compete on price, because my customers in turn didn't give a shit what my back office was running on. Why would they?
That is the situation as it is now in many companies. They are locked into XP for good or ill because they simply could not afford to have Linux tools developed. Like it or not, and while the situation has changed considerably in the least decade or so, but a decision for Linux usually meant one for more freedom and flexibility but also higher cost. That Linux doesn't cost anything doesn't mean much. It changed considerably, but 10 years ago it was a higher risk to migrate completely rather than simply going for the next Windows version.
That has even little to do with managers' inability to see past the next quarter report. For many the choice was just "fold now or deal with the consequences later".