It's a simple equation really. If he stays healthy the next two seasons, he makes $68mm, and hits free agency for a MASSIVE payday in line when new TV deals are coming into play (this is of course assuming the Cowboys don't franchise him a third time, but that seems unlikely given the $50mm price tag and their cap situation that year). If he stays healthy, it was a phenomenal decision. If he doesn't stay healthy, it's all sorts of question marks. Clearly he feels his track record of staying healthy will continue. Me, personally, I feel like that's a big risk to run and regression to the mean (injury-wise) is more likely.
But, man, I like his confidence in betting on himself. He certainly seems confident at least. These things can only be judged by how they look in hindsight, can't be judged from where we sit today. And in my mind it will come down almost exclusively if he stays healthy, because I believe it'll be hard for him to go wrong in this new offense with the weapons he has at his disposal.