I disagree with the OP.
Why?
1) "starting" is not what matters most, minutes played is what matters most! Now if the OP had said "it's time for Sheppard to get more than the 24mpg he's gotten so far", then yeah I would support that. Remember when BCG started Coury all year, but he played <11mpg. Minutes matter (most).
2) "IT" (our current lineup) is working. Why rock the boat, why mess with something that's working. Now if we start having struggles, sure this could/should be something to consider.
3) related to #2, we are about to add another piece (Bradshaw). Let's limit how many things we change at 1 time when we are playing well.
4) Confidence. Some guys need perform better starting, others perform better coming off the bench, and others it doesn't really matter. Obviously both Sheppard and Dillingham can play well coming off the bench!!! Can Wagner? Maybe, maybe not. Again, why rock the boat, if you don't have to. Cal and the other coaches who are with him every day can better assess a player's mental/emotional ability to handle certain changes, than us fans who don't know the kids.
5) Spark. I like that "spark" or "energy" when Sheppard and Dillingham come in (usually together). It actually reminds me of when the "White platoon" would enter the game in Dec 2014, you could just see the exhaustion on the other team's face that we just put in a fresh 5 as good as the 5 we took out and that they were going to be relentless for the next 5 minutes.
Now if Wagner starts playing bad, then yeah it may be worth revisiting. But he had played pretty well the past 2-3 games, including that 28pt game.