Aside from the Jets game, I'm not exactly sure what you are citing in the final four games that made Brady look shaky. And I would ask if you actually watched any of the Broncos games from the second half or if you are just scouting box scores. Because if you watched them, from the Rams game on, Manning never looked right. That he was still able to put up good overall numbers is a tribute to the space between his ears. He had some good games and some awful ones and it certainly didn't appear to be because of a quad injury.
If it was just a quad injury that led to that, would there be all the hullabaloo about if he's coming back next season or not? He's an insanely competitive guy and has a huge salary to collect, never mind chasing the elusive second ring to secure his legacy. I doubt a quad injury would end his career and he's not working out in NO right now to test his quad. I don't think it's at all crazy to wonder if he physically can perform at even the level you are citing over another season. And even those aren't as great as you are making them out to be - a 90.3 rating would slot him #17 in 2014, right behind Joe Flacco and just ahead of Jay Cutler. Also behind such luminary QBs as Ryan Tannehill, Alex Smith and Manning the Jr. A 90 rating isn't what it used to be.
This is to say nothing of their schedule next year. Non division looks like:
Home: BAL, CIN, GB, NE, MIN
Away: CLE, PIT, IND, DET, CHI
So yeah, I'm in Ed's camp. I could easily see them going 9-7 unless Peyton returns to an elite level and they bring the whole band back. And when you're relying on a 39 year old QB with multiple neck surgeries who last time we saw him looked like he lacked the physical strength to advance the ball more than 20 feet in the air, it's past the usual "well if x,y,z happens, any team can falter".
And Vegas odds mean shit dude. Citing those means nothing.