You're welcome to other reasons to avoid C# and .NET, but worrying about their longevity doesn't look like a rational reason.
C# and .NET will endure in one form or another, but all these transformations waste developers' time and make their skills obsolete. No thanks, I'd rather learn something and use it more than a couple of years without having it change under me for no good reason.
My point was that it doesn't matter if c# or .net continue to exist or not, all it matters for me is whether one can count on one's skills being still valuable. My Visual C++, WinAPI (bit WinRT is replacing it) skills are still fine, my C# & .net framework that I learned back in 2006 are nearly useless, because a lot of stuff changed.
Why do companies expect developers to always gobble up whatever they throw over the wall and say "thank you sir, may I have more sir?", as if it's our duty to relearn how to do X for the Nth time, this time with bells and whistles?
More importantly, why do devs like investing in such ephemeral knowledge?
> "as if it's our duty to relearn how to do X for the Nth time, this time with bells and whistles?"
There are a few areas of computing that are fairly stable, if stability of knowledge is your priority, you should seek one of those out.
But in many other areas of computing, our discipline is young and rapidly evolving. Some of that evolution is being driven by a corporate need to sell the next version. But plenty is driven by the rapid evolution in languages and paradigms. And why is that? Because we all know that programming can still be (much) easier, more reliable, more predictable, cheaper, and more fun. This is why many devs, myself included, like investing in "ephemeral" knowledge.
Honestly, I understand that the decay of hard-earned skills is distressing for many people. Not all fields have this characteristic. But in our field, it just comes with the territory.
I don't mind at all when a language is evolving, that's indeed to be expected and manageable, because one can leverage existing knowledge and augment it. But a lot of change is either driven by corporate interests as you mentioned or by fashion and is disruptive, as in replace X with Y. And it's not clear how Y is more robust or faster, usually it's a mixed bag of pluses and minuses.
And Windows Forms still works on the new platforms ( except mobile), so it's not like it broke functionality.
VB 6 Applications still work and that IS a long time ago...