I agree with you. Ash will see a little time at back-up point. But with the emergence of Lindsey, a Demps/Lindsey combo will be very tough.
My thought is that you'll always see either Mac or Demps on the court, with Mac getting ~32 minutes on the ball (and the rest on the bench). He's too good as a facilitator to expect him to sit much more than that, whether he defends well or not.
Those other eight minutes, maybe Ash is on the ball or maybe Demps is on the ball. I assume it'll evolve over time.
In general, I don't think Ash is going to see a lot of time at the point until he's a senior unless he proves to be better than Mac, or unless Mac gets hurt.
A point guard is like a quarterback. The preference is to never switch, unless circumstances force it. NU happens to have perhaps found that four-year point as a true freshman, good for him but less good for those that come after him.