WhatsApp is one example, and if you're trying to imagine your own path, you should be thinking in statistics and probabilities. Imagine if the idea doesn't work out and you need to pivot. I'm not familiar with Erlang at all, but judging by the fact that (1) it was developed by a big telecom company and (2) it's priorities are stability and performance, I don't feel that it drastically changing business logic of an Erlang application is an easy task.
Being an Erlang developer myself: if you don't actually need the stability of a telecom (e.g. restartless upgrades that hold open circuit-based connections), pivoting around the logic of an Erlang app is about as easy as doing so in Ruby/Python/etc.
Though, that being said, I'd actually recommend Elixir, which gives you the same Erlang-ecosystem benefits, but with the key advantage of being 5x easier to convince the average Ruby/Python developer to use.