1) Who are the other players?
Several players who are older than Jansen, who additionally made their first Pro Bowl appearances in the mid- to late stages of their career, or played at a high level as they aged.
Jansen while a good player, has gotten steadily less effective. There was a time where he was a fringe Pro Bowl player. Right now, he's nowhere near that level of play and other players in his age range have maintained and in some cases raised their level of play.
One obvious explanation is the level of wear and tear on his body. For many years he didn't miss a game, and played through several injuries. After a while, that catches up with you. Neither you, nor I know what his off season workout regimen is, however, if you listen to ANY older player, they say to a man that as you get older you have to change how you prepare your body in the offseason.
Now, I think I am giving Jansen the benefit of the doubt by suggesting that maybe he is at the stage where he should try a different approach to his physical conditioninng, RATHER than assuming that his skills are diminished.
He's a good player, but he hasn't looked like the same player. Sorry if that hurts the feelings of some.
Chris Samuels -> Is in superior shape and has arguably improved over his 7 years in the league.
Alan Faneca - 10 year Vet (still dominant)
Walter Jones - 11 yr vet
Mike Wahle - 10 yrs made first Pro Bowl in 2005, his 8th year.
Jeff Hartings - 11 year vet, retired while still dominant. Perenial pro bowl
Tarik Glenn - first pro bowl in 2005 his 7th year - retired at dominant level
Larry Allen - no need to even explain. Dominant, old, and still wins the NFL strongest man competition consistently.
Ray Brown - Dominant player until he was 40, and has himself stated that he prolonged his career by changing how he prepared in the offseason.
Maybe you will complain that Jansen was never this caliber of player. You have your opinion , I have my own. I happen to believe that Jansen at one point was on the cusp of being that kind of player, however, it hasn't materialized for him for whatever reason. Maybe he can revive his career and make a case for being one of the dominant guys at his position, however to many people, a Hall of Famer included, believe that he has not looked like the same player.
AGAIN: EVEN AT HIS CURRENT LEVEL, I still think he is the best RT on our team and deserves to start, however at some point, we can't blindly hang on to this guy and expect him to be dominant into his mid to late 30's when he's not even dominant now.
Parcells decides he would rather risk having cornerback Terence Newman cover Moss man to man. When the Cowboys are in man-to-man coverage, Newman will go where Moss goes. Parcells points out Redskins right tackle Jon Jansen. “This is Jansen getting pushed back,” he says. “He doesn’t look like the player he was a couple of years ago.”
http://www.nytimes.com/2006/10/29/sports/playmagazine/1029play_parcells.html?ei=5088&en=cd475c257228379a&ex=1319778000&partner=rssnyt&emc=rss&pagewanted=all