I've seen no indication that Mack Brown is anywhere near wanting to go into retirement. Thus, I think the jury is still out on whether WM ultimately remains in Austin.
As for Nick going back to the pros, I don't see it happening but there IS precedent for this kind of thing happening. Ray Perkins had the Giants doing pretty good when he left them after the '82 season , and we all know the rest of his story and what he did--he left us to go back.
I think we've set Nick up contractually where he simply would be leaving a boat load of money on the table if he ever chose to leave, and an amount of power and influence over a program that cannot be duplicated--anywhere. The man basically has a lifetime contract if he wants it. Add to that I think his wife and family really like it here, and all arrows point to him remaining here for quite some time, IMO.