You can 100% work your way to a senior position without ever leaving windows. The people who are like that just don't tend to be hanging out on platforms like HN.
Now the number of senior C# engineers in Europe who couldn't fix a broken deploy on IIS or SSL cert problem on a windows server? That is rather high in the windows field too.
To be clear, this was developing software running on *nix environments, and they were all using Mac (or WSL) and the usual open-source *nix dev tools. This is not a case of developers purely targeting Microsoft environments (indeed it would be excusable in that case).
In Europe C# fills the role of Java.
You're just in an American echo chamber.
Now the number of senior C# engineers in Europe who couldn't fix a broken deploy on IIS or SSL cert problem on a windows server? That is rather high in the windows field too.