Remember if OCaml wasn't a mature programming language, maybe Rust would not have happened in first place.
However, I very much disagree with Graham's 2004 assertion[0]:
It's a lot of work to learn a new programming language. And
people don't learn Python because it will get them a job;
they learn it because they genuinely like to program and
aren't satisfied with the languages they already know.
It does not require "a lot of work to learn a new programming language" once a person has fluency with at least one. Actually, the difficulty of learning a new programming language is inversely proportional to how many programming languages the person has already learned. Especially if a new programming language is in the same paradigm category as those already known (Procedural, OOP, FP, etc.).I was a professional software engineer in 2004, when the Graham post was written. To say, "people don't learn Python because it will get them a job ..." was bullshit then just as it is now. The remainder of the quoted sentence is unfounded extrapolation and has the value of same.
But being outside of the mainstream may mean you need to occasionally debug more esoteric stuff: https://gallium.inria.fr/blog/intel-skylake-bug/ I'm sure Jane Street can afford doing that, but I'm not so sure if a small team can.
Because despite them being very open about it, no one else does it, and every distinguished engineer who pushes a weird tech choice will justify and defend it.