This makes no sense at all. Why do you think Witten can’t block a DE or LB, or run a pass pattern or catch a ball with changes to the offense? So what if Witten played in the old offense - so did Zeke and Dak and Frederick and Martin and Tyron Smith and Jarwin and Schultz and the majority of the players that will play on the Cowboys offense this season. When teams make changes they don’t fire everyone and get new players.
As for the “check down city” idea, playing 40% of the snaps means he will sit 60% of the time - how does the 40% dictate the offense over the 60%? Do you think they are going to run one offense 60% of the time, and another 40%? Plus, much of the 40% will likely be in 2 TE sets, meaning the TE playing the majority of the snaps And getting the majority of the targets will still be in the game as well, and Witten will not be the only TE option.