I think it'd be a tough decision - D-Will's a definite upgrade over Rondo at the PG, but you might have to take a step back to get timing right w/ a new Point guard. Though I don't think thats a huge concern - we already saw this team function relatively well w/o Rajon.
The Biggest issue would be contracts. We've got Rondo locked up at a REALLY good price until 14/15, whereas you've got no guarantee on Deron past next season. And if we were able to resign him he will definitely cost between 5-8 million dollars per season more than Rondo.
I agree I don't think the Cs would do it without a contract extension from Williams. But I also think there something to the perception of both players as it pertains to the future of the Cs, and how they rebuild post-Big 3.
My sense is Rondo is seen as a great player, but a stubborn guy with a number of flaws. On the flip side, Williams seems to be viewed as maybe the best player in the game in his 20s outside of Lebron, yet a guy who cares deeply about competing and winning (i.e. not taking his talents to South Beach).
I think it would be easier to build around Williams, much as I love Rondo and feel he's utterly unique.