I'd like to see GW stick around. This defensive squad is built around his "non-traditional" type of defense. Mark Barron is the perfect example.
That said, I think only a HC that was already a HC and has some "gravitas" would retain Williams. A young guy without previous head coaching experience may be intimidated by someone like Williams who's been there and done that. Williams may not be keen on having an inexperienced HC offering input on his players and schemes.
I really think the "D" has run in to a combination of being exhausted and getting to the point where they had no faith that the offense would even do anything to keep things close. Let's face it, other than the 21 points the O put up in the first half of that one game (Miami?), they've done virtually nothing and I suppose that as human beings, the D sort of said, "screw it, I'm saving my body for next year.