Unless you're actually CPU-bound, there's never been a better time to use it. A lot of the benefits of Rails apply as much to LLMs as humans.

It's battle-tested, there are millions of lines of existing code using common patterns and libraries. It's still the gold standard for testability, with RSpec and factory_bot you'll have the nicest test setup with concise, human-friendly specs (especially if you're in the camp of verifying LLM work). Solid queue, cache, Active Storage with Turbo Frames give you all the benefits of single-page interactivity without the JS slop. Code size is a fraction of an equivalent JS setup, which is always nice for LLMs, went from close to 80k lines of JS to ~10k of Rails just recently on an internal project.

Historically, the main complaint was always "it's hard to hire for Rails" or "I don't know Ruby". But looking at the last 10 years of JS, I'm not sure the vast majority of JS slingers knew anything about Javascript either. And since everybody's vibing now anyway, why venture to the minefield that is the JS ecosystem.