Julian,
I also have a Python background :)
Elm will not replace Python anytime soon as a
general purpose language.
Python has way more libraries and documentation, plus some problems are just easier to solve with an imperative approach.
That being said, Elm needs not replace Python.
I see myself still doing small, utility scripts in python and serious work in Elm for years to come.
Elm has a very solid design so, after it reaches 1.0 I see it start to eat a lot from the jobs of other languages.
It will get alternative backends and this might open the door for even more applications.
As for Haskell, Erlang, Elixir... Elm was designed to be friendlier than either of those.
So, it makes no sense to learn Haskell first if you want to work in Elm and it makes a lot of sense to learn Elm first if you want to end up using Haskell.