1,000 yards and 10+ TD's aren't relevant. Those only matter in fantasy. He was as good in 2016 as he ever was. Whenever he was healthy, and needed, he was great. This Dez under performed in 2016 narrative is false to anyone who watched the games. He simply was used as more of a decoy, while the offense ran through Elliott. Dez was better in 2016 than 2013 in my opinion, its just his stats weren't, because the offense was different, and he was used differently.
2015 had Weeden and Cassel at QB, which makes Prescott look like Brees by comparison. Dez seemed to care a little less that year, and I'll admit the tape wasn't as good, but I don't think it had anything to do with a decline of skills. I think even Dez himself would admit he wasn't really all there in 2015, as the entire team seemed to throw in the towel a bit when Romo went down.
Is it possible Dez is hitting a decline phase? Sure, but its just as possible, in my opinion more so, he's not, and a change of scenery will prove that.