Lots of people are reading too much into Heupel "bringing Milton in" IMO.
I think CJH knows that his best chance of early success and momentum lies with scoring lots of points and that all hinges on the QB position. I don't think he brought Milton in "to start" but rather to increase the competition in the hopes it produces a really good starter. I also think he knew Salter was unlikely to be on the roster.
I would love to see Milton get consistent and become the starter. His arm is amazing and D's could never sleep on the QB run. But IMO, Hooker currently has the best combination of risk, running ability, and passing ability. Unless Maurer or Milton overcome their inconsistency, Hooker will start unless he stumbles in August in which case it will be Bailey.