I don't think RoR usage has declined (maybe as evidenced by the number of "I'm a newb" comments here?) but rather think it's become mainstream so it's simply missing the hype it had when it was the new hipster technology.
They just happen to be successful, and popular.
I don't think the trope of Ruby being old and boring is such a bad thing either, it just means it's stabilised and has a strong ecosystem that requires little to no effort to get set up. And it still gets a lot of love with every Christmas release adding something desirable and new.
I also think it will require some immense innovation or paradigm shift to unseat Ruby/Rails as a de-facto framework for rapid web prototyping. I would still kick off a project with Rails in favour of trying to early-adopt some new approach to development.