That F# has no native GUI framework is the reason why I stick with C#. When coding something with a GUI, the majority of my development time is typically spent in the GUI layer. The rest is some more or less straightforward object relational mapping with some validation an calculations in a business layer where I use Linq to emulate functional programming in C#.

Adding F# to the game would only complicate the scenario, because I would need to connect the F# interfaces somewhere to C# anyway, requiring an additional layer of mappings in many cases.