Well, let's face it: most of the Z warriors are only around because they had lost to Goku in the past. The series repeatedly uses the theme of having a former enemy team up with Goku (Yamucha, Tenshinhan and Chouzu, Piccolo, Vegeta, Buu), but that meant they accumulated a lot of characters who weren't strong enough to fight the current enemy, especially from Frieza on.
DBZ then, was centred not on the left-over characters from DB, but on Goku, Gohan, and Vegeta. Clearly, the original idea was for Gohan to eventually become the greatest of them all, but even this falls apart after the Cell Games. It's pretty obvious that everything that came after that was done just to milk more money out of the franchise, and, from a story-telling perspective, was a mistake.