> One of the big reasons why FP languages have so little penetration is because the advocacy usually feels like someone trying to talk you into a religion.

That's exactly what worked for OOP—have we collectively forgotten just how hard OOP was pushed everywhere 15–20 years ago? Way more aggressive than any FP advocacy I've seen, and I've seen a lot. When I was learning to program every single book and tutorial pushed OOP as the right way to program; procedural programming was outmoded and functional programming was either not mentioned or confused with procedural programming.

I still have to deal with the fallout from OOP evangalism; I've had colleagues who unironically use "object-oriented" as a synonym for "good programming" and managers who believe design patterns are software engineering.