Bears' issues don't have to have a single source. I am not trying to defend Trestman or claim he is cut out to be a NFl HC - some people are better OCs than HCs, after all. But to religiously defend a player who has done nothing else but disillusion people with his talent, at all organizations and all HC and OCs he's ever been with, is quite misguided imo ... bordering on blind favoritism, I'd say.
No one can deny he has the physical talent/attributes needed to be a QB. What separates the great and the average QB, however, hides int he intangibles, the things you can't measure for, like heart, leadership, winning/never-say-die mentality, etc, etc. That's where Cutler is as lacking as your average Joe out there.
He looks completely disinterested in the game as if he'd rather be somewhere else. There is no fire in the guy to make his teammates believe he can be their leader and they can rally up around him thru thick and thin. It's simply difficult to take the guy seriously, as a leader - all he's been talking about for 10 years now is how he should get better and in his 30s he is still playing at a level suitable for 2nd year player (or even a rookie), not a seasoned vet. Compare his aloof behavior, especially after yet another loss or poor performance, to how the Brady's, Mannings, Breeses of the world - he is not even close to the passion and strong feelings those guys show. For them football is a passion, for Cutler, more like a job, a chore. This affects what you project to those around you and how they perceive you - as a leader they'd be willing to punish their bodies for, or as an over-paid schmuck who just doesn't care as long as the pay-checks keep coming. Projections and perceptions in a sport like football matters a ton, you could even say it's everything - a coach can loose his players but so can a QB.