That's a question of probability. If I'm the GM of the Ravens, I think there is a slightly higher probability that a guy like Flacco could be replaced than a guy like Matt Ryan. It has to be a switch that makes sense, though, and the better the QB talent the less likely a GM is to find a superior talent available. It's rare for it to happen, but it does happen on occasion. Donovan McNabb got replaced by Michael Vick. Both of those guys are/were pretty good. Occasionally the planets do align and a franchise QB like Peyton Manning is dumped in favor of a rookie like Andrew Luck--that probably should not have happened like that, but the team was in turmoil, so. Then there's Alex Smith *flinch* who is better than many around here think he is, and he got benched for Kaepernick--controversial to say the least.
I think it's even fair to say that Green Bay was grooming Aaron Rodgers while Brett Favre was still setting records. Honestly, I don't think the search for QB talent should ever stop even when you have Captain Franchise out there. The guy could break his neck on any given day, then what?...