And the latest version is Unity based. Also, I'd say the only thing that prevents Enterbrain to do a similar move, is simply that they don't even have nearly the same significance & reach as Unity. RPG Maker always was a niche product, even if it a popular one at that. But you don't really see big commercial game releases that are based on it like you do with Unity.
We'll have to see how this Unity thing plays out, but if this isn't going to be the engine's downfall, then it will become a new norm for others to follow.