[thinkharderdev]

I think to gain adoption any general purpose language needs a clear niche to gain a foothold. If you are choosing a tech stack then choosing a language that is not already widely adopted is always a risk. The tooling is usually inferior, there are not necessarily high-quality libraries in domains you care about and you pay a cost in training new employees in a new tech stack.

Java was an OO, garbage collected language that supported (sort of...) hot swapping code. Go was a reasonably performant, garbage collected language that compiled to static binaries and has great cross-platform support. Some grad student decided to write Spark in Scala so you had to use it for Spark jobs Rust is a systems programming language with zero-cost abstractions and no GC that allow you to write mostly memory-safe code.

What is the killer feature of OCaml that makes it worth the investment?

[yawaramin]

OCaml has many (almost all) of the advantages that you attributed to the other languages, and adds many more besides. What makes them 'killer features' for those languages but not for OCaml?

[thinkharderdev]

Nothing really, but they are mainstream languages and OCaml is (currently) not. So if you really need one of those then there is already a more widely adopted option that gives it to you. It's not that OCaml is bad (I like it a lot) it's that to adopt it you need a really compelling reason to do so.