I'd rather have a code base I'm going to be working on in a language I haven't learned yet, than having it be in C or C++ if it's of any significant size.
Learning a new language is small thing, all things considered, especially if it's a well designed one like Haskell.
Spending a week or two getting familiar with the way things are done in a language, and then gradually become effective in it and the specific codebase I would be working on for me at least would beat having to work in an environment with 50 years worth of irreconcilable technical debt inherent to the language.
I agree with you fully, but I've also tried to onboard people to F# and Haskell and... unless you're the self selecting person that enjoys (typed) functional programming, the pushback you get from the other ~95% of developers is extremely strong.
If your stack is FP-ish, and you hire FP-ish developers, it's fine. But having non-FP devs write Haskell? Maybe I've been unlucky, but it's near impossible in my experience.
Spending a week or two getting familiar with the way things are done in a language, and then gradually become effective in it and the specific codebase I would be working on for me at least would beat having to work in an environment with 50 years worth of irreconcilable technical debt inherent to the language.