Witten isn't part of the offensive problem. He's a good blocker and is still an affective pass catcher even though he doesn't stretch the field often. I do think there's an opportunity to give another TE more snaps in long down situations, while Witten still primarily plays on 1st, 2nd, and shorter 3rd downs. But he's still a good player that deserves to be on the field a good amount.
Here's what I would propose. Witten played 98 percent of the Cowboys snaps last season, that's probably high at this stage. Lower it to 70% or 75% of snaps for Witten and 30% or 25% for a younger TE. That equates to about 12 to 14 less snaps a game provided you have a TE that you feel could make plays down the field. To me this is the best scenario as you get Witten on the field in situations he excels at, while you get more down the field play making ability on plays when you need more yards.