If Favre had come right out after the 07-08 season and said, "I'm 100% committed to coming back next year and figuring out how we can take that next step and win a Super Bowl," they would have let him. No doubt.
They would have had little choice. They were so close in 07, and Favre had a fantastic year, he was MVP runner up. To push him away and cut or trade him if he had fully committed would have looked insane at the time. I know it's easy now to say move on with Rodgers, but at the time Rodgers was a complete unknown and Favre was a legend coming off a huge year in which we were a play away from the Super Bowl. Even the Packers weren't 100% sure what they had in Rodgers as evidenced by spending a 2nd rounder on Brohm as insurance.
Despite all that, I do think the organization was ready to move on with Rodgers, but they needed a reason to, they couldn't do it on their own. Favre gave them the reason they needed. They didn't want to put up with and no longer needed to put up with an entire offseason of a wishy washy Favre playing a game of 'will he or won't he.' I think Favre felt like he was entitled to play that game and would have taken all the way up until training camp every year to decide had he been able to. TT and MM weren't going to have that.
Revisionist history, if Favre had come out right away after the 2007-2008 season and announced a return, I think Rodgers would have gutted it out for 1 more year as his backup. If the organization had simply caved and relegated Rodgers to the bench after Favre said "changed my mind" in the summer, I think Rodgers would have been a lot more likely to ask for a trade right away. But we'll never know for sure.