Super is ending because its reached the logical end of the Dragon Ball franchise.
-By beating Buu, they beat the strongest mortal being in Universe 7.
-They then fought the gods of Universe 7
-They then fought and beat their twined universe, Universe 6.
-And now they are fighting the strongest guys from across the entire multiverse, and Jiren is the guy so strong hes stronger then the gods of destruction.
At this point, the only things stronger then Jiren are the Angels, and Grand Priest, and none of them are evil, and none of them particularly have the attitude to give into Goku's demands for an honest fight, meaning he really will never get the chance to fight any of them except Whis, who is always just fucking around when fighting Goku.
Dragon Ball has gone from a story about a boy who knows some martial arts, and is oddly resistant to bullets, to having that kid beat Demon King Piccolo, beating Freeza, the ruler of the universe, to fighting the gods, and now surpassing them.
It's done, let it go. Goku reached the power ceiling, and can now just fuck off and live a happy life training for simple shits and giggles, and not because something is threatening to destroy the universe or w/e.