Huh? I don't think there is any doubt he would change starters if he felt the team would benefit. He benched Bledsoe for Brady and won a Super Bowl; he's not leery of media-created QB controversies.BB is not going to give Grier or Zappe a start. His experiences in CLE with Kosar and in NE with Bledsoe/Brady undoubtedly make him extremely leery of anything close to a real QB controversy. For better or worse BB believes a QB controversy is a negative to his team and he will not engage in one.
Barring an injury Mac will start the rest of the year.
I think he doesn't believe that Zappe or Grier are better options than Mac Jones. Sometimes it is that simple.