I think it's very possible that Bowles, for example, would be the new DC. Or another guy already on the staff.
Why would you come to Dallas knowing that if BP coaches this year, that's probably it and then he retires?
That's just not much job security in a profession that already has very little job security.
Heck, if I were a prominent DC on another team, I'd only take the job if the payoff was huge (i.e., what I could make in 1 year with Dallas would be equivalent to 2-3 years somewhere else), or I was potentially slated as the new head coach (assuming my performance was good) once BP retired.
I think the most LIKELY scenario is that the new DC comes from within the organization.
That said, I'd prefer several choices outside the organization . . . .