If the Titans don't franchise Young, I bet he plays for another team at the end of his contract (as a starter) by his own choice. I don't think he enjoys playing for Fisher, and I also don't think Fisher backs Young the way a head coach should back a starting QB.
McNair was never an elite "passer" in the NFL. Yes, his passing was better at the end of his career than the beginning, but he was never on Manning and/or Brady's level with passing. He had the ability, but the offense limited his opportunities.
You brought up McNair not me, but Young is ahead of McNair at this same stage in their NFL careers. McNair sat on the bench for several years before he got the chance at starting. Once McNair got his chance, he was thought of very similar as Young is now. Everyone called for O'Donnel (sp) over McNair when McNair finally got his shot. (I was a McNair fan BTW.) I was at the Titan's game when McNair got knocked out of the game and the crowd cheered because it meant O'Donnel was coming into the game.