What has been the point of having SA bring Manu off the bench for so many years when he was clearly one of their best players?
It's about chemistry and fit. Utah will have a couple of playmakers in the their starting-5 with Hayward and Burks or Hood. You have those two, Favors and even Gobert wanting to score. There are only so many shots to go around. Remember how we used to complain about Hayward being invisible in the 1st a lot of games? That all magically changed once Trey went to the bench.
If the vet PG was a 3PT shooter (like Fisher used to be alongside Kobe), then yes, he'd be a good fit as a starter. If the vet PG is just a better version of Trey, then he's a better fit as a 6th man when you need that player's scoring with most of the starters out of the game.