I think it's important to look at how twitter got there though. They'd been using just Rails for a long time before introducing Scala. At that point, it's fair

I think it's important to look at how twitter got there though. They'd been using just Rails for a long time before introducing Scala. At that point, it's fair to say they weren't a 10 dev startup anymore. This, imo, is part of having developers choose the best language for the job. They chose ruby to get their mvp out quickly and be able to grow their userbase. After that, they pinpointed trouble areas in their architecture and made pragmatic choices in fixing up those areas. While I suppose it's possible to "let developers pick the best tool for the job" and end up with 5 different languages in your stack, most good developers are likely to steer clear of such an endgame, unless the trade-offs are very, very clear.