You do realize Witten was the 7th rated tight end last year? 7th in receptions, 12th in yards and 10th in tds (only 3 behind the much heralded Ertz, Gronkowski and Kelce)?
His average per catch was 80th, but that's more a function of the team not going downfield much to anybody, since Dak was under pressure a lot. And Witten is Dak's security blanket, Witten hangs around close to the line of scrimmage so Dak can dump the ball off to Witten to avoid a loss of yardage. He's not going downfield because of the offensive scheme, not Witten's lack of speed. He never had better than an 11.9 average per catch, he was never fast, but speed doesn't help you if you can't get open.
And you have to make your tight end a focus of the offense for him to have the great stats, and the Cowboys are set up for Elliot to be the focus, tight end is not going to be a big part of your offense in that case.
But the Cowboys do need to develop Hanna or Gathers or whoever, no doubt about that, Witten won't play forever. And drafting a tight end in later rounds wouldn't hurt, Gronkowski was a second round pick. Jimmy Graham was a third round pick. So there's tight end talent in later rounds.
Like receiver, imho, tight end is not at the top of the list for needs on this team...