.NET is huge in enterprise. There are easily billion plus distinct lines of .NET (C# or VB) code running on companies we don't even know the name of.
Now with .NET Core, .NET Foundation and the push for open sourcing, the rate of new C# lines written per day is probably increasing even more.
C# jobs are plenty. Much more than some hyped languages around here. And it shows no sign of slowing down.
By the way, C# is no longer tightly associated with Windows for a few years already. It's common for C# code written today to empower a microservices fleet deployed over a large kubernetes infrastructure.