Whoever wins the Chaffetz seat will surely vote the same way on every single matter that comes before the congress over the next two years. Curtis, Ainge, etc, what difference does it make? The winner will be quite literally the least important person in congress and will just vote whatever way the caucus leader says every time for the duration of the term.
On the other hand, this is a once-in-a-lifetime shot for Tanner Ainge: a rare open seat in one of the safest districts in the nation. Nobody votes in these primaries, so there is a good chance it will be close. If Danny Ainge loses out on Hayward, he can just sign someone else and the Celtics will be hardly worse off (or maybe somehow better). If he manages to get Hayward and his son loses a close election... that has to hurt him, right?