I would put it at less than 1%. I was very hopeful in January, but him playing in their spring game and being "the guy" in their spring practices killed that hope. Why would the coaching staff give him all these reps unless he gave them assurances that he's staying? Imagine we were in Arizona's shoes, wouldn't we be screaming to high heaven if the coaching staff gave all of their QB reps in the spring to a player who might leave?
I'm hoping I'm wrong because all of the reports I've read are about how the Illinois QBs are not ready and Tate would take their 2019 season into a new level.
I am less concerned about Tate playing in Spring, because the decision to leave is his own. However, he is growing a ton in the system (which would be his biggest reason to leave). He starting to see the growth, learn how to read defenses in the way that Sumlin teaches and improve. They are starting to give him the green light to scramble at make plays when he sees fit. These are all reasons I think he will stay.