Nostrademons said "for programmers that can use GC, there are already very good solutions out there", which I understood to mean "the advantages of most ML-family languages over other languages that require GC are smaller than the advantages of Rust over other languages that do not require GC". Which I was questioning.
I wasn't thinking specifically of ML-family languages, though they could be included.
Rather, if you are in a problem domain like web-development or server-side microservices where a GC is fine, there are lots of decent options for programming languages. Python, Ruby, or PHP. Go. Any of the JVM languages - Java, Scala, Clojure, Kotlin. Swift or Objective-C. dot-NET. Many of these have had continuous attention over the last twenty years, they've got major corporate backers, and so a lot of the recent research in PL theory gets ported over to them.
If you are in a problem domain where you can't use GC - like computer graphics, games, databases, information retrieval, operating systems, or embedded - you have basically one option. C++. C++ has gotten a much-welcome facelift recently with C++11/14/17, but the core of the language is still 40 years old, and the language as a whole makes serious compromises (like memory safety) for backwards-compatibility. The excitement about Rust largely stems from its competition being C++; if you pit Rust against say Python or ES6 in the domains in which the latter are used, it's very much "Why would I use this?"
I don't know if I'd pick Java as a comparison point because the ecosystem is unimaginably large, so it's hard to compare to Haskell/OCaml. A better one might be Golang or ruby.
I think ML languages just never made the case that other languages are bad enough for the particular thing someone is doing now, e.g. a web app. If you hit a NULL, you get an exception, track it down, probably not a huge issue. The best case could be made for ML on security grounds, but unfortunately nobody cares about security.
On the other hand, people were convinced that C/C++ were bad after decades of people saying so. But people didn't feel like they had an alternative until rust came along.