AKA mobile and Windows.
.Net is cross platform on mobile. Outside of it, if your project ever need to run in anything that isn't Windows, or communicate with anything that isn't Microsoft, then you can't use .Net. That makes it fit for enterprise CRUDs and not much else.
I mean it is honestly by far the best framework I've uses to communicate to everything from 40 year old hardware to modern web services and APIs.
As far as "run on anything that isn't windows" I'll grant you, that's a very emerging area, but you can run .net (or some flavor it) on a lot of different platforms.
Hell I've got a raspberryPi running windows 10 IOT with .net running on it, and it works like a top.
And statements like "If Microsoft doesn't pour a billion dollars into getting .Net ready for Linux in 2018, .Net is over." say's to me, you don't really have your finger on the pulse on the enterprise ecosystem at all.
You'll be hard pressed to find a language that has breadth and depth that .net has. Couple that with the new push to Open source, from an enterprise standpoint, it's a very attractive platform.
Windows 10 IOT is basically Raspberry Pi and Allwinner A64 only at this point, which excludes most of the ARM SBCs out there.
The problem with both these platforms is they are horribly I/O limited, to the tune of 1 full USB port worth of bandwidth. Other SBCs commonly push 4x that or more!
These programs can do anything, just like any other code in any other language, and the protocols (like HTTP or UDP) to talk between processes or programs have nothing to do with the code itself, which gets compiled down to the same machine level instructions anyway regardless of what language you use.
So what communication protocol are you using that .NET can't talk to? In Enterprise it's almost always SOAP or REST and .NET has some of if not the best technologies for producing or consuming both.
That's just plain wrong. Not only might .NET be a good option for a backend system (which can, of course, communicate with any platform), but people have been using .NET on non-windows systems via Mono for many years