I would say there are several languages that straddle the line. AFAIK there exist 50+ people teams that use i.e. ocaml (Jane street, Facebook), haskell (galloi

I would say there are several languages that straddle the line. AFAIK there exist 50+ people teams that use i.e. ocaml (Jane street, Facebook), haskell (gallois, Facebook, Google), erlang (whatsapp, pivotal) I would say that beyond i.e. 10 developers you suddenly need to have onboarding solved. You need to be able to teach your new hires. You need to have community beyond your workgroup to ask questions. You need decent library support. I would say it is beyond 50 where your leads start to worry you didn't `go with IBM` so to speak. You suddenly can't just hire an expert. And you can't coast on the fact your programmers find the language familliar. Like, I joined 4 people team working in clojure. It was fine. You mostly could see "oh, this is where the macro magic starts, and here I need to grok protocols first and here I need to grok transducers first". Then my manager got nervous that he couldn't hire new people fast enough so we had to do a python rewrite :-/ Different team, different project, 60+ people, python. You'd think, it is just python. But you never know when there is a metaclass hiding behind an inheritance chain. There are parts, where we might have single person understand the code. Sometimes they left year ago.