There are two distinct sets of skills involved in being a developer:
- The Generic relating to how to analyse, design and construct decent applications
- The specific relating to the toolset you use to implement those applications
There are - or certainly have been - people who exist at a level of "coder" and whose skillset is limited to a single language but, I think, progressively less so as any programmer is (or at least should be) expected to be more rounded.
If you have the former and an ability to adapt to new variations or alternative toolsets then you shouldn't have a problem.
Prior to that was VB.OLD and some Turbo Pascal and prior to that a variety of stuff.
I've also, over the past 25 years, looked at and learnt about a variety of different industries and businesses as I endeavour to learn the domains as I develop solutions for them. I also have a moderately decent understanding (though not to a sysadmin standard) systems and networks and making things talk to each other - simply from the need to be able to have the platforms to develop on.
Right here, right now, .NET is a vast and capable platform with decent languages and tools and as good a place to be employed as anywhere - more important (I think) are the broader skills, unit-testing, agile methodologies, application of patterns (understanding of good principals like inversion of control) and good practice (use of VCS and build servers and the like) - these are portable skills that need to be kept up to date as things develop.
I could go on (in case you hadn't noticed) - but the simple answer is that so long as you grasp that you are a "developer working on the .NET platform" rather than a ".NET developer" (or worse a "C# developer") you'll be fine.