Recap: Ruby on Rails is not dying, it just isn’t the cool kid anymore.
IMO every language essentially goes through this, gets popular for some period and then slows down to become either mainstream or it falls away. I'd personally say Ruby went mainstream, it definitely isn't fringe and it isn't the new hot stuff but real companies rely on it daily.
Truth be told Node.js, Laravel and Phoenix are steadily chipping away at its piece of the cake.