I know it is a copycat league but a big athletic TE like Gronk (who has missed a lot of games) is not the only formula. Witten is so good at finding or creating space, drawing coverage, and either making a catch for a moderate gain or opening things up for other receivers to make big dynamic plays. Not every play is going to go for big yards.
I hear the argument that a more athletic TE would rack up yards after the catch and stretch the defense and create mismatches all over the place. That is one model. What Witten does, even at an older age and a step or two slower, is another. I'm not saying I wouldn't like to see a more athletic TE but Witten seems to keep beating them out. If you think he is getting the legend treatment then look at Romo. They cut him and ate his $20 million cap hit because Dak basically beat him out and I am not sure there is a player Jerry loves more.
Of the 7 passes Witten caught on third down in 2017 that did not go for a first down only twice was he stopped a yard short (both led to a punt). He helps control the ball and control the clock which Dallas would rather do than score fast TD's from all over the field.
Maybe Witten is overpaid at this point in his career but his $6.5 million cap hit is not crippling the Cowboys. Romo's cap hit is bigger.
Disclaimer: I like Witten and Romo. I am lukewarm on Dak but I am not calling for him to be replaced. I like the dynamic he adds with his running ability but he can't do that for long so I'd like to see him develop more as a passer. I ultimately think Dallas can win with him.