I’m assuming you have stats to back up that Joe T. struggled as a Starter?
You can't look at these things in a vacuum. The Stat that matters when Joe T was starting was wins and losses, and there weren't enough wins and too many losses. I love Joe T, and I think he has starter talent. He isn't a sure-fire-build-your-program-around guy, but can play. So for every player not named Keegan Murray or Luka Garza, it is about complimentary talents.
I love onions and I also love strawberries, but, I don't love the two together. I love strawberries more than I like peppers, but peppers pair so much better than strawberries do. Same analogy in basketball. Playing Joe T with JBo left us with 2 small guards and JBo had to play the 2 because JoeT is a much better place then a 2 because of his inconsistent shooting. The size and lack of quickness of JBo really left our back court as a defensive liability. But for this team to have the best chance to win games, it was better to keep JBo's shooting rather than Joe T's quickness.
Enter Perkins, who is much bigger, can defend bigger guards, can rebound, can handle, can shoot better than Joe T, and so him pairing with JBo really allows either to play the point and either to play the 2 depending on the match up. Perk is the best player on the team to take defending guards off the dribble and finish at the rim.
But saying that doesn't mean that Joe T doesn't have unique talents as well, as he does. That is why I think he plays so much better with Sandfort, because he can still play the 1, but Sandfort can handle the bigger guards and small forwards, and Sandfort can shoot. I think Joe T pairs really well with Perk as well for the same reasons, although without JBo on the floor you lose a court-stretcher, experience and the clutch gene. Jbo is really good at getting into the offense, with speed but not out of control, and getting good shots.
I love the fact that Joe T has a great role and is really playing well in it. And I give him all the credit in the world for taking off and running with it, and Fran for, however he did it, to get him to believe on the importance of it.