Also, higher performance translates to lower machine costs. Our server costs would easily be 4x if we ran rails. given that elixir is 80-90% as productive for writing code in the short term, its a fair deal. Over the long term, we've found elixir to be more maintainable since there's very little magic and the immutable data structures and lack of magic make it super easy to debug.

Thanks to the performance of our system, we've also managed to get by without setting up caching. (we'll probably need it eventually but our performance gains have come mostly though fine tuning our sql queries. Ecto gives us way better control over the output sql than active record or sequalize)

People who pay for those extra boxes? Not everybody has your budget (especially when they're building things themselves, on the side, so they aren't even paid).

Scaling vertically can take you a long long way before you actually have to start separating services.