I cant recall the last time a team went to the playoffs three seasons in a row and let their QB walk with no viable replacement behind him.
You dont make a change just to make a change, you only make a change if its an upgrade and even the most ardent Dak critics on this site have acknowledged that there is statistically nobody on the roster that is on Dak's level.
That is why I dont believe the Cowboys will let Dak walk, at worse they will give him a short term contract of 2-3 yrs to give them time to find his replacement.