The key distinguisher there is static typing. Which is how Facebook manages to make do with PHP, and Dropbox with Python. By adding top annotations for a static type-checker...
It would be interesting to hear a comparison from someone using https://crystal-lang.org/ at scale. It's basically Ruby + types, which would make it the closest for isolating that one feature. It can't run Rails, but there are very similar web frameworks available.
It misses other benefits of static typing -> being able to compile to efficient code. Also Sorbet is still not popular enough to apply to many gems - it's a lot of work to implement it at the moment.
It's not Ruby + types, it just looks like it at first glance. The semantics are all diffrent (for good reasons).
I very much doubt that anyone is using Crystal at the scale Ruby/Rails is used yet. It's still far away from a 1.0 release and when I tried out 0.23 it had several issues, major ones for me were lacklustre debugging support and the type checker needing to load the whole AST into memory. Concurrency story was also not very strong but I know people were working on it actively at least.
Anyway, I wrote a non-trivial project in Crystal and overall really enjoyed it. Improving the debugging tooling would go a long way towards making it a real option.
