True, but that's an argument to move on to other, less popular (for now) statically typed languages which are better than C and have REPLs.
We could debate whether it's wise to move on from C (I think in many situations it is), whether it's likely there will be more industry adoption of $MY_FAVORITE_LANGUAGE, but that's a different debate.
Also, if we keep favoring C (or Java, or whatever) because they are mainstream, then other languages will never become mainstream, and we will be stuck forever coding in horrible languages. Who wants that?
Well, we can debate how mainstream Haskell is, but especially if you mean ML in the sense that does not include also OCaml, then ML definitely is not a mainstream language.
OCaml might be mainstream to armchair FP developers on HN and language theorists at Facebook, but it remains a fairly unpopular choice for developing software outside academia. Even Haskell is pretty unpopular relative to the amount of software developed daily.
Would Scala be mainstream? OCaml or Haskell probably not.