I think you're right about the separation of ecosystems into .NET vs. everything else. I often idly run a find-in-page for .NET/C# on the Who's Hiring threads just to monitor uptake in the start-up world and rarely get more than about 6 hits in 100s of posts.

I ended up in a C# role by chance coming from a non-software background and it can often feel like a separate world; so many discussions in the other ecosystems just feel less relevant since C# is so batteries included you rarely have to venture outside the confines of the officially mandated environment.

While there's a lot of stuff in the corporate environment on Windows servers and old full-framework code there's also a healthy and growing Linux first community using .NET.

I don't know that I'd recommend it particularly from outside if not trying to start a .NET role specifically, I don't think (based on the languages you list having experience with) it will help you learn anything particularly new, in the way that learning a functional language does for someone with an OOP background. Though LINQ is quite a nice way for people unfamiliar with functional programming to ease into it from an OOP environment.

With .NET 5 / dotnet core the getting started is easier than it used to be (thanks to dotnet new https://dotnet.microsoft.com/learn/dotnet/hello-world-tutori...) and a lot more like other languages/frameworks, but without a specific aim I think it's going to be hard to be motivated to get in-depth with the environment.

To be clear I wouldn't want to work in any other language or framework (though React with functional components and TypeScript comes pretty close to the developer experience quality of C#/.NET) but I'm just not sure how porous the ecosystem boundary is and how transferable the skills are.