The criticism is mainly that he didn't work on his route running, and judging by the rounding of his routes, that criticism seems justified. When he started to slow down, that might've helped him stay relevant.
Perhaps. But "slowing down" was not what proved Dez's undoing. It was injuries. The knee, the foot, then finally the achilles. The thing is Dez could roll out of bed right now and post up a DB on an 8 yard in cut and if the ball is delivered on time he will catch it 98 out of a 100 times. On at least 20 out of those he will break the tackle of that DB and turn the 8 yard in cut into a 20+ yard play. Maybe even a house call. Speed was never a big part of his game.. So losing it really would not have played that big a part without the injuries.
Don't even get me started on what he could do in the red zone. A throw it up and go git it receiver is something this offense has needed in close since the day he was let go. I have often maintained that this version of Dez would mesh much better with this version of Dak. Early on Dak simply wasn't throwing the ball to you if you didn't have at least a yard of separation. Now he trusts himself to place the ball where only his guy can get it.. and in Dez's case that means putting the ball up where only HE can get it. Covered or not.. Dez is coming down with 80% of those.
Last but not least was his alleged "diva" persona. Accurate or not, the mediots always painted Dez as a problem child. Maybe some of it was deserved but as we learned on several occasions, a lot of it wasn't. But it fit the narrative so they ran with it. I think it dogs him to this day and I think he reached a point in his life where he wasn't interested in fighting it anymore. At some point a person gets tired of swimming upstream. Dez probably still has 20 million bucks in the bank.. So he has no need to.