Something you have to consider though is that, within the context of the saga, Goku and Vegeta coming back didn't really change much.Dragonball continueing after the Buu saga kind of killed that idea. But in the Buu saga it felt like a last ditch effort. Gohan was established to be stronger than Goku was at the time, and Goku's ace in the hole, the Super Saiyan 3 form, drained his power so much that he couldn't even maintain it for a full fight.(which is why he had to rely on the spirit bomb). It never felt to me like Goku was taking the place of the next generation. It simply felt like a way to wrap up his character. The idea of having everyone Goku has met, helped and saved feeding into a spirit bomb (a move that has never worked) and finally going from an eternal student to a teacher of a new generation is a really solid way to wrap up his character. That's why I feel like the Buu Saga is such a good bookend on the series as a whole.