#1 his long term health is his decision. It is not, nor should it be, a factor to the Cowboys. If Romo is healthy, and wants to play, you don't hold him out because it might effect his health later down the line. He makes that choice, not the Cowboys.
#2 Romo is what is best for the team. Right now. Today. He's the better QB. Period. What Dak is doing is great. It can't be said enough. But anyone thinking that Dak is as good as Romo right now is simply crazy.
If those guys want the best chance to win, right now, then they want their franchise QB back. They want Romo in there. He's the best shot at winning now.
Going forward into the future, obviously Dak is the better option. He's younger. He's got longer to develop into a big timer. But for some of the older guys on the team, like Witten, they don't have the time left to wait for Dak to develop the experience or QB savvy that will be needed to win it all.
That's why when Romo is healthy Romo will still be the guy.