Well, by switching to F# you get much better type inference, discriminated union types, pattern-matching, explicit tail-recursion, serializable types by default

Well, by switching to F# you get much better type inference, discriminated union types, pattern-matching, explicit tail-recursion, serializable types by default, and type providers (and that's not even a comprehensive list). I suppose it depends what type of app you're building as to whether those are worth anything to you -- but in the areas where F# is rapidly getting more popular (data science, numerical/algorithm-heavy work, distributed computing) they do in fact make you much more productive. A good chunk of that productivity gain results from being able to easily and quickly write type-safe code and express algorithms without a great deal of syntactic overhead. A JavaScript developer might be more productive than either a C# or F# developer depending on what kind of app he/she was writing; overall I don't think that would be the case, especially once you factor in maintenance costs of the code over the long-term.