Here's how I see it:
Look at our roster and incoming recruiting class (which is sparse right now, but I think when it's all said and done, we should land some nice jucos to fill the holes). If Bill goes and it's not someone internal, you can (a) kiss that sparse recruiting class -and any jucos - good bye and (b) the new guy has ZERO time to get anything cobbled together. Couple both those things with our current depth chart (specifically on defense) and the next coach is just set up to rough ridin' fail. I'm also worried that we'd get some rough ridin' pro-style offensive guy or something and eff Sams.
If it is an internal canidate, I still think we will lose some of those jucos - specifically to Weis - and I just think that in comparison to the year after, it would be more difficult given the new starters that we'll have to have on defense.
If he waits until after next year, he'll probably be returning a lot of starters (on offense and defense) which will (a) be more attractive to an outside canidate and (b)give the new guy a better starting point.
Let me also add that this year we'd have way too much competition for viable canidates.