> In F# ... 350k lines of C# were rewritten in 30K of F# code.

Not discounting the rest of your statements (I love FP for all the reasons), but I really despise - and discount - statements like the one above. I feel they are disingenuous:

"This large-ish program that grew over time with evolving needs and uses was rewritten at a later date once the requirements and use-cases were fully understood and - amazingly - we were able to greatly simplify the code!"

It probably could have been rewritten in BASIC and still resulted in fewer LOC. That's not to discount the null checks, "zero bugs", or other factors. My favorite part of programming in Haskell is that 99% of the time if it compiles it just works, and I can refactor code + add features without concern.