Deciding on what language to use on a new project and rewriting a large app are two different things. It's much harder to sell a rewrite than it is to sell using a different language on a greenfield project.

If you know Elixir and are confident using it, you'll understand its benefits and when best to use it. I work for a dev shop where we use Elixir for client work. Elixir is performant and you end up making the most out of your server resources. That would be great for clients who end up having huge traffic. But more important than performance, I find it easier to maintain Elixir code largely because it's a functional language. Lastly, I think Elixir's Lisp-inspired macros are a great way to write reusable libraries/modules.

In essence, I'm more productive with Elixir than I am with Ruby even though I used Ruby for much longer. I find it a more pleasant experience maintaining Elixir code than Ruby code.