Here's how I look at it. It's very rare for a team to win a SB with an average QB. Typically their QBRs are pretty high. I saw a graphic or tweet IIRC that only one team in the modern era has won the SB with a QBR (a QB's season long QBR) under 50. And that was Manning in Denver. Talk about the perfect game manager (with declining skills) on a team with an elite defense. Their O was still decent/average (ranked 16th). Their D was #1.
JB's QBR now is like 47, and that's with one of the best OLs in the league. Our D (pts per game allowed) is like 16th. Even if our D gets back up to like 10, that's still not a SB winning combination. I also ask what is more likely, finding a QB that will be top 10ish (60 or higher QBR), or creating a #1 D to offset a sub 50 QBR. IMO, finding a 60ish QBR guy is probably the easier road.
I really love JB as a person. Awesome guy, hard worker, etc.. I'd love for him to improve and become that guy. I just see several very "fundamental" limitations to his game, that I'm not sure will improve all that much over the course of a season (or even two). My biggest fear is that we get stuck in a fog of mediocrity, trying to make things work for a few years. Purely my opinion, but if I don't see significant improvement by the last quarter of the season, I'm probably moving on.