It was Goku and Vegeta show‚ but now apparently even Vegeta is turning to a fodder.
It was Goku and Burma adventure at first. Then Goku, Krilin, Yamcha rivalry, followed by Goku, Muten Roshi, and Tien rivalry, Goku-Picollo, Goku- Vegeta, and so on.
Tbh, DB and DBZ are pretty good in giving side character screentime, like when Goku was locked in healing water while Krilin, Vegeta, Gohan and Picollo get most of the screen time in Freeza Saga. Or when Tien could hold Cell for a while.
But in DB Super, it is only Goku Show (with Vegeta sometimes step in). And I don't see this will be different soon considering we get a potential antagonist that tied to Goku's father.
Anw, for Sabo, I think there are only 2 possibilities.
Either he was captured and sentenced to death. Or there was a huge destruction that happened and Sabo was supposed to be dead in that destruction.