The chances returning for his senior year turns him into a lottery pick are much lower than him getting injured or having a bad season. I don't think a lot of people realize how much an extra year of age hurts players in the draft. Grant isn't likely gonna grow another inch or change his game that much. The problem he has is he is a very polished player and the only real thing he can add is a 3 point shot.
It makes zero sense for him to return his senior year. The only reason he should possibly return is for legacy and that my friends would be a stupid personal choice as much as we'd all love him for it. Even if he ends up being drafted in the second round it's a better business decision.