I don't think it's a given Dez will work harder and make them all work harder. He might, but focus and hard work were never traits Dez displayed as a player. His emotional intensity was nice, and I don't fault him for that, but he also was known for being divisive in the meeting rooms, showing up late for meetings and practices, and the Cowboys covering up a lot of his other transgressions. At one time, he required a paid baby-sitter from the Cowboys.
Now, Dez may have grown up and show up and work hard. His circumstances surely point that way, but the world is full of people down to their last chance who still do not change their stripes.
The Cowboys know Dez better than anyone. My guess it wasn't just Witten and Garrett who he wore out his welcome with. I would love the talented Dez to be given a chance, but if he's lost 1/3 of that talent, I am not sure Dez is worth the headaches he could bring. Yes, there's a chance he won't bring them, but is he worth bringing the old Dez minus his talent into the locker room? I don't think so. Unfortunately for Dez, he made a bed he has to lie in.