IMO C# is picking up a lot of small things that make it incrementally better than it was before, and at some point it's not worth the hassle to go to F#. F# features are gradually added to C# so C# developers have time to pick them up as well.
You cannot add the best features of F# to C# without making it a fundamentally different language. You're not gonna make C# into a type-inferred FP language by bolting on some random half-understood bits anymore than you can make a cat jump better by gluing rabbit legs onto it.
It's the core paradigm and philosophy that makes it great, and if anything, what I've seen hold it back from its potential is insufficient support requiring essentially importing C# idioms into the language just to interop with existing libraries.