C# definitely will remain the number 1, and FP will remain used in a minority of places, until more people get exposed to it earlier in their cursus.

The advantages are in the idioms, immutability by default, null not being legit value by default, emphasis on correctness, advanced type system etc.

The issue is often that people doing C# won't gain enough FP practice to see where F# brings most advantages, try to use it as a OO language for really short time and ditch it because tooling and concepts are not the same as what they are used to.

I'm confident though that adoption is rising and that few years from now most .net shops will get exposure to some amount of F# code, and that people who have been doing C# for so long will eventually pick up any functional language, which in turn will make F# really appealing to them.

I'm happy C# is supporting constructs such as local functions and some basic pattern matching (although type tests are probably the worst use case of pattern matching as seen in ML languages), but I'm also concerned the language is evolving toward more and more complicated syntax and rules (see all the kinds of scoping rules for variables and switch statement) and so little things geared toward correctness (readonly locals, non nil references?).

C# focuses on alleviating developer pain, but sometimes making the choice of enabling to do the wrong thing more readily.

The only way I see FP becoming more popular would be if some popular libraries came up that were purely functional and showed the strength of that approach. I think I understand the benefits of FP but it struggle to see how it would fit into the current library ecosystem. There are not many people who have the luxury of building a codebase from scratch. Most of us need to use other libraries.