If the Koroks were successful, we don't know. There has never been a game to prove or disprove their success. So judging on the fact that they haven't been proven successful yet, it is safe to assume that they were not successful. Same principle as "innocent until proven guilty" because you start out innocent, then you must be proven guilty.
Anyway, we are talking many, many generations here for the Korok's plan to work. They are trying to build trees that will expand into entire countries or even continents. That's not something that happens in a couple hundred years and surely not something that happened by ST, since I know you mentioned the setup of its regions. There's no way that ST's Hyrule is the Korok's work. Even FSA seems a little to close to the times of the other Zelda games to say that its on another Hyrule created by the Koroks. So really, their efforts will never quite make sense if they show up as successful in a future Zelda title since logically, what they are trying to do would take thousands of years.