Crystal aims to replace Ruby but at Railsconf and Ruby meetups I go to Elixir/Pheonix is mentioned 100x more than Crystal is.
I don't think it aims to replace ruby (they don't even claim any kind of "compatibility" level with ruby, more like "inspired by ruby" I would think), but...I wish it would replace it :)
I don't know if Elixir is mainstream or not, but there are a lot of companies using Elixir in production (including the company I work for). You can find the list of companies at https://github.com/doomspork/elixir-companies