Elixir/Erlang's emphasis on uptime/fault-tolerance/scalability/concurrency (admittedly at the cost of performance and the abstraction of a VM) is perfectly suited to web/mobile use-cases.
On paper, Pheonix should be the default choice for large and small web/mobile MVC applications. I think the question of if that becomes the case is directly tied to adoption of Elixir.
The performance comparison matters if you start comparing nginx, and maybe even the Go/Rust frameworks, but it absolutely blows Rails out of the water for performance without sacrificing any of the high-level abstraction goodness that makes Rails appealing. No reason not to move to Elixir unless you cant hack functional programming IMO
On paper, Pheonix should be the default choice for large and small web/mobile MVC applications. I think the question of if that becomes the case is directly tied to adoption of Elixir.