Where did you hear that? I always assumed that it was because of the fans since that makes much more sense. From the beginning of Z, Gohan is clearly hyped up to be a prodigy of some sorts, a successor. And then by the end of Z, he doesn't even fight anymore. As an author, why hype up a character to be important and then make him irrelevant by the end of the series? Hell, Gohan's last fight was one of the most embarrassing fights in the series. Even in the 13th movie(the only movie where his full potential is unlocked) he's stronger than Goku and he didn't even hit Hirudegarn once. Even if it's not canon, the fact is that Toriyama was aware Gohan was gonna eat dirt.