It's easy to just say "sit down and shut up" but it's not that simple in reality. Braxton would have every right to be pretty upset. Dude's worked hard and played pretty well. If I were him, knowing I could start on plenty of teams in the NFL, I wouldn't like being sat on the bench. Who knows how that looks and what that becomes.
Plus, Poles has shown now with the Fields trade that he likes to do right by his guys. It now would be fully in-character for Poles to move Brax along and maybe spend a pick on a guy to back up the 9th overall pick. If Ryan Bates can get a 5th round pick then you'd have to think Brax can get at least a 3rd, maybe even a 2nd from a team looking to bolster their line and open up their first round pick to spent on another position instead.