I think there are a few key differences. The "old RoR" way assumed that all behavior is built by the server through ruby, until client side behavior was needed

I think there are a few key differences. The "old RoR" way assumed that all behavior is built by the server through ruby, until client side behavior was needed. Then, that was built in CoffeeScript. (or JS if you rejected omakase.) The React way is to assume you will have client side behavior, but write it as if it is client-side and the server will compile that into a html that can be hydrated with behavior without intervention. Even with that "magic," it is easier to reason through if you already have a mental model of the browser.