I'd suggest looking at Elixir/Phoenix if you have a problem with Rails that you think Elixir/Phoenix might be able to solve. Could be scale problems, reliability, real-time communication, etc.
I wouldn't suggest replacing your entire Rails app with Phoenix right away. If possible start with a service that may be having issues and migrate that over if possible.
I'd suggest looking at Elixir/Phoenix if you have a problem with Rails that you think Elixir/Phoenix might be able to solve. Could be scale problems, reliability, real-time communication, etc.
I wouldn't suggest replacing your entire Rails app with Phoenix right away. If possible start with a service that may be having issues and migrate that over if possible.