I worked in .Net shops for a few years, back in the days before .Net MVC. Part of the reason people ended up (and maybe still end up) stuck in .Net might be because back then, as soon as you tried stepping outside of the .Net webforms box (i.e. tried to do anything remotely interesting) things because exponentially complex. Keeping all of the .Net classes and foibles in my brain during my day job was enough for me not to want to learn the intricacies of a different system in my off time.
That was years ago though... and since then I've happily moved on to being self-employed and using OSS.
Yeah Microsoft tends to deprecate and replace frameworks every few years. Usually it's an improvement, but they really keep you on an API treadmill.
Eventually I got tired of it and went back to primarily Win32 development for customer-installed apps and Linux for everything else. I feel like I can focus on quality rather than quantity much better now.
That was years ago though... and since then I've happily moved on to being self-employed and using OSS.