I dunno. Jane Street started using OCaml back in 2002. Consider the world of programming languages back then: no Rust, of course...but, like Java wouldn't even have generics for another two years! At the time - and for a long time after - OCaml's offering of power/performance was pretty unbeatable. It arguably still is, but I think there's at least an argument these days. I'm not sure exactly when that argument became viable, but it would've been long-after Jane Street had already built up a pretty huge codebase and a pretty crack team of OCaml engineers.