According to NFL Network Insider Ian Rapoport, the four-year contract extension through 2021 -- which Witten signed Tuesday and was first reported by ESPN -- will act as a "cap related move to lower his number for 2017." Witten, who is turning 35 in just two months, would theoretically be under contract until he is 39.
While there is no doubt the ironman tight end could make it that far, this is, as Rapoport mentioned, for bookkeeping. Witten was set to count $12.262 million against the team's cap this upcoming season.
There is little doubt that Witten, already the franchise leader in receptions (1,089), second in yards and fourth in touchdowns, would have retired a Dallas Cowboy anyway. Kicking some of his money down the road makes it almost a guarantee.