One thing Elixir definitely needs is a killer app or framework that really plays to the strengths of its Erlang underpinnings. A web framework is probably not that app/framework.

Those already exist with extreme maturity in other ecosystems. Scaling the web framework part is pretty easy since, unless you royally screwed up your design, you should be able to just add more stateless web heads to service more requests and you almost always should end up ultimately bottlenecked at the persistence layer, not in the request router/processor.

I just don't know what that killer app/framework will be.