It's an interesting question. I mean, on the one hand, if Dak walks away then that's incentive for the team to look for another QB in the draft. I don't think that his agent would consider this in his best interests.
But the real question, in my mind is, what do other teams see in Dak? Do they see a young QB who can't throw or do they see a young QB who takes care of the ball and who can be effective in the right offense? Honestly, I don't think we know the answer to that.
I don't think it's true that Dak will not play out his contract. I don't see any advantage to that line of thought for Dak. It doesn't help him in future negotiatitions, should he become an FA. It doesn't help him with the Cowboys, it doesn't serve him in his time towards retirement and it certainly doesn't help him in his development. I don't see him holding out.
JMO