> "I was just wondering if Erlang would be considered a "proven language"."

Erlang is proven in production for server and telephony apps.

> "From my (limited) Erlang experience, Erlang & Elixir have the same core functionality."

Depends on what you mean by 'core functionality'. If you mean the functionality provided by Beam VM or OTP, then there's a case to be made for that. However, the library ecosystem for Elixir is separate from the one for Erlang.

To use an alternate example, Java is definitely a 'production ready' language for enterprise software. However, if I write a new language for the JVM, it's not automatically 'production ready'. In most cases it's the library ecosystem which is under evaluation, not the platform the language runs on.

Erlang has been in use for decades, so yes. For many years, and possibly still true, nearly every phone call in the world went through Erlang code. Yes it's proven.

As I remember WhatsApp did quite a heavy modification of Erlang/VM, i.e. not a typical/idiomatic Erlang use case I believe.