I agree to some extent, but haskell pureness for example , which I take to refer to monads and other concepts from category theory in haskell, is actually very very abstract; the "constraint" of pureness (IO monad) just leads to the design of more consistent and very general interfaces; once you become familiar with these interfaces you are quicker than you would otherwise be, also haskell's type inference help alot with expressiveness vs. dynamic languages. In fact, personally my road to really learning haskell was through converting shell scripts to haskell, a task typically left to dynamic languages but which haskell might actually be uniquely suited for contrary to convential wisdom due to aforementioned factors.

Not saying haskell is some kind of panacea, doesn't run in as many environments as javascript, and can't handle programs that require very high performance at very low latencies due to garbage collection.

For web dev, the library situation seems very mature to me.