It depends on how you measure sustainable. What would happen to Elixir without its BDFL? Has Elixir achieved escape velocity and will continue? Rust was pushed out of the Mozilla nest and managed to live on. I don't see a similar capacity for Elixir handling such existential risk. Are there succession plans? Is there mentoring among members of the core?
I also think that it's necessary to point out that just because OTP facilitates resilience doesn't mean that the system you'll build with it will exhibit that behavior. You have to design your system to use OTP effectively. Easier said than done. I've seen Elixir systems reach a critical failure state following maximum restarts of its internal processes. However, at least Elixir has the potential of being hardened. It's just not going to act as expected without a lot of up front wisdom applied.