If all you are doing is writing API then you already don't need 50% of what Rails provides. If you are not into ORMs then you don't really need anything Rails provides at all.

I'm not expert in Ruby, but I'd say there's definitely a few wins in typescript (types, performance, async). But the biggest wins have nothing to do with language differences, but rather not having to worry about that because you're using one language. If teams are large and split between api and frontend, then perhaps less of an issue. But if you're doing anything fullstack, to be able to use one language, everywhere with mostly the same ecosystem...well it's kind of what Java wanted to be.

And I say this coming from a python/django background. And ORM is one big thing that Rails/Django definitely are clear leaders in.