There's already alternative implementations, with somewhat different featuresets:
- Akkoma (more minimalist than Mastodon; Elixir in the backend): https://akkoma.social/
- Misskey (much more feature-rich than Mastodon; Typescript in the backend; extremely Japanese): https://misskey-hub.net/en/
Having said that, frankly, I doubt that the Ruby ecosystem will fall apart from this. There's too many non-volunteers whose wage depends on Ruby to keep going...