With salary cap things are different for teams that have a great player but need to rebuild. We have a lot of recent examples of players encouraged to retire from a rebuilding team, who just had to have another season. Jerry Rice comes to mind leaving the 49ers for Oakland. Not a great finish for a brilliant career, but the days of a player spending his whole career in one place have become truly rare.
Brett has had to live up to the Bart Starr legend / comparison in Green Bay. For whatever reason, he had to come back. Brett shares a legacy of one-team quarterbacks with the likes of Terry Bradshaw, Roger Staubach, Joe Montana, Dan Marino and John Elway. By going past his prime he risks being in the ranks of Jim Plunkett, Jeff Hostetler, Kurt Warner and others who were great, but finished on the decline on different teams.