Well first, Elixir isn't purely functional. It's got side effects all over the place, it just doesn't have mutable variables.
Second... While I love static typing as much as the next guy, there's definitely value in immutable data in a dynamic language. You also get Dialyzer in Elixir/Erlang, which is an interesting alternative to the usual type systems you find in Haskell/Rust/etc. in that it focusses on finding places where there's provably a problem, rather than where it's not provably correct.
> there's definitely value in immutable data in a dynamic language.
Sure, but there's even more value in immutable data in a statically typed language, because of all the advantages that come with such a language (automated refactoring, better performances, tooling, etc...).
I've been working with the BEAM (via Erlang) for over a decade, and with Elixir for almost 1.5 years, and I'd still give up, well, probably my current job if I could get a fully Hindley-Milner Typed Elixir...
Second... While I love static typing as much as the next guy, there's definitely value in immutable data in a dynamic language. You also get Dialyzer in Elixir/Erlang, which is an interesting alternative to the usual type systems you find in Haskell/Rust/etc. in that it focusses on finding places where there's provably a problem, rather than where it's not provably correct.