We know who Uub is, but he really just comes out of nowhere in Z and is strong enough to surprise Goku.
Then Goku leaves to go train him, presumably as part of the next generation of Earth's Z fighters.
But this robs us, and Uub, of what Goku got.
Experience. And a journey of discovery and learning.
A huge point of Goku's story is him gaining experience, learning right from wrong, becoming less naive, experiencing loss, and growing as a person. Uub immediately being trained by Goku to become extremely strong skips all of that.
When Goku got trained by the various masters, a big part of it was him going out there and getting experience as well.
Potentially in the future, we might get something post Z, and it will probably just be a time skip to showing Uub being super strong, much older, and getting close to Goku in strength and then off he goes for an adventure, joining others, after that.
I don't want that. I would want to have a Dragon Ball style adventure with Uub so we watch him grow as a person, not just in strength.
I think it would be much better for it to be changed so he meets Goku during Super. He wants to train under Goku, but Goku says he needs to go out there and get experience, like he did.
Then we get a whole arc of Uub doing so, going out there, experiencing the world as Goku did, learning how he learned.
Then in end of Z they have their fight, Goku realizes how much he's grown, and then agrees to train him.
Maybe I'm wrong, but I think that since Toriyama already made end of Z, and now he's gone, nobody would ever rewrite or change it because that would be thought of as disrespectful to him and his work.