Just to turn your own example against you for a moment: Mono is dead. If you were using it for your cross-platform WinForms desktop app, as my team was, you're now stuck with no updates and no migration path to MS' new offering, MAUI (though it still doesn't run on Linux, and nor did the several UI frameworks in between). At least Mono is still getting patches. They even recently did a stable build after a long hiatus.
I believe the cause of Mono's death was a combination of MS buying out the company where many Mono devs worked (Xamarin), and the huge rearchitecting MS did for .NET Core. The latter being something that on paper sounds great for openness and cross-platform support—but in practice, it hasn't turned out that way. The .NET team is mostly MS and they still play favourites with Windows (and increasingly Android, which is fair) and Visual Studio.
Eh - I don't really know that dead is the right term, and I think it mostly served it's purpose (It's actually still getting fairly regular commits, but I agree that it's no longer keeping pace with dot net core)
I would also draw a pretty clear distinction between C# the language, and something like WinForms.
And that's really the whole point - implementations differ in functionality exposed and features worked on (hell - just the compatibility issues and differences between clang/GCC/MSVC is a great example). The corporate implementation is usually the most feature full because it has the most resources poured into it.
That's not a problem. That's the community being able to take advantage of those resources. If/when the company stops being a useful partner, they're free to ditch them and fork (and this is historically how things like Linux/OpenBSD/Gnu tooling exist...)
So again - if you have a real problem with using the corporate release, use the open versions. Contribute to them.
But I don't find it a compelling argument to say that just because a language is sponsored by a company we should avoid it. Doubly so if the corporate version is licensed well.
I believe the cause of Mono's death was a combination of MS buying out the company where many Mono devs worked (Xamarin), and the huge rearchitecting MS did for .NET Core. The latter being something that on paper sounds great for openness and cross-platform support—but in practice, it hasn't turned out that way. The .NET team is mostly MS and they still play favourites with Windows (and increasingly Android, which is fair) and Visual Studio.