There is no "rebuild" needed at Texas. Their roster is full of high level talent. All they need is a coach that can get the most out of them. You don't think Urban would be able to do that? When Urban talks, those guys are going to listen.
If they don't he'll kick them off the team. It will only take 1 recruiting cycle to fill any of those spots.
If Urban goes to Texas, he'll have them in the CFP by year 2.
I'm not sure if anyone can challenge what Alabama and Clemson are doing right now (I don't think OSU is on their level either) so a Natty might not be possible, but Urban would give them the best chance to get it.