Aaron Rogers is in the last year of his rookie contract, if they don't go with him this year, they run the risk of him walking via free agency and then they're out a 3 year investment and potential Favre replacement.
I get where they're coming from, but we're talking about one of the greatest QB's in the history of the frickin' game, coming off of one of his best seasons ever. It's a lot to give up for Aaron The-3rd-Year-Back-Up Rogers.
Don't forget that the Packers have a long history of trading away Favre's back-up QBs: Kurt Warner, Aaron Brooks, Matt Hassleback, Mark Brunell, Doug Pederson, & Steve Bono are all former Favre back-ups that were traded away during Favre's tenure. It honestly wouldn't surprise me to see them deal Rogers instead of Favre.