I feel like developer experience of Discord is the only argument you need when talking about whether Elixir can scale economically. If a company can do it on venture capital, it's well within the grasp of FANMAG.
Edit: That said, squeezing every last drop of performance out of a server by writing everything in Rust is something that a FANMAG can well afford and may make more sense at the 100mil+ concurrent users scale, but it wouldn't be economical to start a project this way without certainty the capacity would be used.
https://blog.discord.com/scaling-elixir-f9b8e1e7c29b?gi=ecb5...
Edit: That said, squeezing every last drop of performance out of a server by writing everything in Rust is something that a FANMAG can well afford and may make more sense at the 100mil+ concurrent users scale, but it wouldn't be economical to start a project this way without certainty the capacity would be used.