As I recall, Montana WAS forced out (with Young in the wings). And they were phasing out Jerry too. By that time, Jerry was leaving a 3rd rate team to join another 3rd rate team (neither the 9ers nor the Raiders were too good), and had a couple good years in Silver and Black. As did Ronnie Lott, when he decided he wasn't done when the 9ers said he was.
In the end, if a guy wants to keep playing, who cares about the legacy? He wants to play. It's more than a business, it's a game too. I don't think you can say the reputations of the players mentioned above are in any way diminished by the years they spent elsewhere. Same with Marcus Allen. They remain great players.
Still, the Favre fiasco was handled poorly (understatement) by both Farve and the Packers, and the media over-reported it, making it a bigger story (outside of green bay) than it really deserved to be. I'm not a Packer fan -- I don't care!