If the Pats could trade for say Matt Ryan and let him sit on the bench for three to four years and not totally screw up the salary cap, I say go for it. Otherwise, the Young reference is useless since it was before free agency and the cap and the 49ers could have the luxury of having two starting QBs on the roster. Could never happen today.
The Packers' situation was different too in that Favre wasn't coming of an unanimous MVP season. The Packers were declining (they went 4-12 in 2005 which was the year Rodgers was drafted and 8-8 in 2006) and the Packers were looking towards the future without Favre because they were in rebuild mode anyway. If they were coming off a 14-2 season with a lot of young players infused already like the Pats were, the Packers may not have drafted Rodgers. But they figured they might have to replace Favre by the time they are good again and took the risk.