To me the biggest mistake the Patriots made was NOT resigning Wes Welker last year. Prior to that Welker and Brady had a thing going that was almost unstoppable. They would move the chains and eventually eat you up with drives you couldn't stop. If you go back and look at the history of the Patriots over the last 12 years (4 before Welker, 6 with, and 2 without) you can see that the addition of Welker jumped the Patriots scoring dramatically. Prior to Welker Brady and Patriots put up 24 points on average. For the 6 years Welker played on the Patriots the scoring average is 30 points a game (6 per game more). Then after Welker leaves their scoring drops from 33.9 in 2012 to 27.9 in 2013 (drop of 6 per game) And this year is has dropped to 20 per game.
Then if you go look at the Bronco's scoring before and after you will find the same thing. Peyton put up 30 a game before he got Welker. Last year with Welker he scored 36 per game. Once again a 6 point increase with Welker. Without Welker to start the season the Broncos have been struggling and only averaging 25 point per game.
It would have been smart for the Patriots to keep him for a few more years to give Brady a real chance at another trophy.