You really need to reread this post.
You prove that Exum doesn't need a markedly improved shot! When they sag off him he can rush for the TD.
HOW? Ex[lain? How does he go for the TD when the defenders don't guard him to the 3p line and instead give themselves the space required to react and neutralize his advantage of speed? When they sag off of him when he's off-ball they double other players, they interrupt more passing lanes, etc. without him being able to punish them for it by hitting the open jumpers. If he can't hit the jumpshot he's a huge liability on offense. There is a reason why guards that can't shoot are becoming more and more expendable in the league.
There really aren't many ways for a PG who cannot shoot to be successful in the league. He needs to be a threat with ball in his hands creating offense and he needs to go to the line to bump up his efficiency because of inability to do it from 3p land. For this to happen with Exum, he needs two main things - handle and off-the-dribble mid-range shot. He cannot be a PG who has insecure and turnover prone handle. He cannot be a PG in the league without having the dribbling skills to survive against pressure. He also needs to develop mid-range game off the dribble at least to the point where he would be a threat from there so defenders wouldn't always take his lane to the basket.