“Source [partially] available” and “open source” are generally considered distinct concepts these days. See for instance [0] or even look up what Google’s dictionary says about the term.
[0] https://en.m.wikipedia.org/wiki/The_Open_Source_Definition
This alone makes your "uncommon" claim void, and there are other examples outside of the Windows ecosystem equally well-rooted in proprietory tools.
Now if MS had their own language (they did...) that was closed source (it was...), the argument would be more akin to this Wolfram debate. But MS learned that wouldn’t fly, and adopted an OSS strategy.
>> Visual studio
> I don't think that's a strong argument
Lovely talking to you.
I would say Windows development in Visual Studio is by far uncommon when you compare with technologies like Python, C (with GCC or Clang), Java (OpenJDK), JavaScript, etc.
that is simply not true and a rather insular view. i build large complex systems, and all of it is done with proprietary software. in fact, it is practically impossible to do my job exclusively with open source software. most open source software is completely unreliable and ill-fitted for many domains. and just try getting good support for use cases only found in industrial use from most open source developers. i have tried, and it never works out. the response is usually "you figure it out and let us know".
furthermore, a lot of proprietary software is fairly open source in that you get access to a fair amount of source code. and it isn't dwindling at all.