NFL Net reported that Tomlin said the Steelers will not scrimmage any other teams which means all they have are 3 preseason games to get competitive reps for QBs.
Ben barely played in preseason so there were plenty of reps for backups, but not this year. Steelers don't have that luxury. They need to make this a 2 QB race almost immediately to give them most reps, then they need to pick a starter by the 2nd preseason game at the latest so that guy can then get major reps.
I generally think it's good to give rookie QBs a redshirt but if you are going to start a rookie, there are a few absolute musts
1. You must be able to protect him. A QB only learns bad habits if he's running for his life. Tons of QBs were ruined by this
2. He must have a decent grasp of the playbook. Not all of it but enough to be effective
3. In practice, he has to at least be close to as good or better than the other QB option. If they other guy is much better, you can lose the locker room because you are telling the team you are throwing the season away to train your QB.
4. If he's struggling, you have to pull him. Again, he won't learn if he's frustrated and feeling pressure to perform. Don't be afraid to sit him and let him watch and then play him again in a few weeks if he has good practices.
Ben checked all of those boxes. The OL was very good. The playbook was simple because it was run first and play action. And Ben flashed huge talent from day 1, then when Maddox got hurt, there was no reason not to play Ben. Despite that, Faneca was still a bit salty about a rookie starting at QB. Ben was good immediately removed all questions of who should start.
The steelers OL could still be rough, especially to start the season. If the OL looks spotty in preseason then you have to start Trubisky.
If the OL can actually protect then it's game on.
I didn't like the Pickett selection. But the choice is made and if Pickett performs close to Trubisky, and the OL is OK, then I think you start Pickett.