DBZ was always meant to eventually transform gohan into the main character, with goku slowly fading down until his death, and gohan holy shit moment with cell making him the ultimate fighter... However to toriyama's surprise gohan's popularity did not pick up as expected, the series lost popularity and well... The story came back to goku to be the one killing majin Buu... There's even an interview of toriyama on the subject #truestory