Your first two sentences don't make sense to me.
For the most part first round QB's don't sit, so you're right about that, even if they play sparingly.
But sometimes first rounders do sit.
First rounders Daunte Culpepper, Philip Rivers, Carson Palmer, Chad Pennington and a couple more all sat. Pat Mahomes played in only one game his rookie year, so for all intents and purposes he sat too.
Ben was drafted in the first round. According to what I read the Steelers intention was to have him sit,
Maddox's injury changed the Steelers' original plan for Roethlisberger, which was for him to sit on the bench or play sparingly during the first season or two in order to learn the team's system. Instead, he started the third game of the season against the Miami Dolphins, finishing the 13–3 victory with 12 of 22 completions, 163 passing yards, one touchdown, and an interception.
Trub was named the starter in 2022. My opinion is that if Trubisky balled out he'd continue to be starter going into 2023. But it's only an opinion that can't be proven or disproven as is yours.