Korea has caught up to Japan with professional leagues per se.
So the outlook will only get better.
Taiwan remains the only country outside the top that doesn't have a professional league. So when the players finish school, its either go else where, or stop playing.
Korea's junior development and Taiwan has always been on par.
But Taiwan's will idle from likely 15 years old due to lack of playing future, while Korea can continue and that makes a big difference.
Competition outlook is not just important for future perspective on players, but also important for development.
I still remember I have a video of Shin somewhere, where she was still a tiny kid, coming on to hit with some champion playing during some ITTF event (kind of like a half time show).