Modern. Net is portable and fast, yes. But .NET framework is (mostly) not portable and it's tied to the version of Windows is on. We'll see when Microsoft decides to EOL it but so far it has not been announced. It's not getting more than security updates now, as far as I know.
We're converting everything we can (mostly web apps) to modern .Net (formerly core) so it can run on Linux. I may be stuck on Windows at work for the office apps, but my code runs on Linux.