So if the fan fic didn't exist, what would you have to say? Nothing. This is essentially the same thing going on with Game of Thrones right now. The fans thought of theories and concepts that predate the official source. In Game of Thrones, R + L = J, which means, Lyanna and Reagar Targaryen have a child, whose secretly Jon Snow. Just because the fans thought of it first does not mean that the author borrowed the idea. Like calm the fuck down.
Oh and Toriyama has recently stated in an interview that he just found out that fans continue to read and study the official source, that means US. In no way does that imply he knows about an Evil Goku from some other fan website.
Dragonball Super carries all the traits that the original Dragonball carried, but without all the violence. It's everything Dragonball was and continues to be. Just because it doesn't portray Gohan as this ultimate, incredible fighter, doesn't mean it's not worth watching. It's funny, enjoyable and will continue to be so.