People have been predicting Jason Witten's imminent decline since 2007 and the Dallas Cowboys themselves tried to draft a replacement for him two years ago.
There's simply no question that Jason Witten is one of the greatest Tight Ends to ever play the game. If you look at receiving records for tight ends, the only name that sits above him with any consistency is Tony Gonzalez. If you look at receivers in general, there's still only one player younger than him with more receiving yards - Larry FItzgerald. Calvin Johnson remains about 100 yards behind him as of the end of the 2014 season.