If the Bears defense keep them under 30, I consider it a pretty good win.
I don't think it's specifically the Bears D thats susceptible to competent QB play - EVERYONE is susceptible to competent QB play. Good QBs carve up defenses, there's almost nothing a defense can do to stop a good QB from moving the ball and scoring points. The way the game is called now - emphasis on the safety of the QB, more rigorous enforcement of illegal contact, the sophistication of play calling and route running - it actually makes it virtually impossible to shut down an offense with an effective QB. It's what the NFL has been angling towards for over 20 years. You're not completely helpless, but the deck is stacked against you and you have to hope for turnovers, sacks, and turning as many drives into field goals rather than touchdowns.
All the more reason why it's absolutely crucial to get a star QB and a good offense. It is the bare minimum to being successful in the modern NFL. Occasionally you get a couple freak teams that can be elite with an exceptional defense and a middling offense, but you are fighting against the current. And there's no reason to fight the way things are going. For some reason, though, the Bears are stuck with it.