Comment by cprecioso

I see your point, but I also think a smaller, more focused language can be useful for teams; in order to enforce best practices.

Think Elm: it's impossible to produce side-effects or mutate objects, cause the language simply does not have the facilities to do so. The guarantees Elm gives you would be hard to maintain if it was just a library which had to deal with JS's constant dynamicness.

Moreover this guarantees do not only improve developer experience; but also compiler optimizations.