There's probably a sampling bias, because of the attitude that one should learn Haskell "to be a better programmer". The FP universe is far broader than the pure FPs. Need I remind: Twitter went from RoR to Scala, ostensibly for practical (performance) purposes, though with hindsight they could have figured out how to scale RoR. So it's hard to argue that nobody has gone the other way strictly for "practical" reasons.

It's also hard to argue that Whatsapp hasn't been wildly successful at scaling to a billion users on a miniscule engineering team in spite of "impracticality of FP".

> So it's hard to argue that nobody has gone the other way strictly for "practical" reasons.

You've made explicit another confounding factor: a language is more than its syntax - you have to consider the entire ecosystem and constraints being operated under - including tooling and libraries

I think there are some subtleties hidden in the word 'practical'. There are differences between:

My company has 200 Java engineers, and a dozen systems written in Java; it's not practical to migrate everything to <FP language>.

Spring Boot solves so many things out of the box that I'd have to implement myself in <FP language>, which isn't practical.

FP has a lot of features but it's not practical to maintain a large complex codebase using those features.