Joe Johnson is a superstar, but he's not a ball-dominant player like LeBron is for the Cavs. In other words, he's replaceable. I think it can be argued that Crawford was more valuable to Atlanta than Joe Johnson was this year.
As for rebuilding, I don't think that's necessary. If I was Danny Ferry, I would offer Mo to them in a salary dump, just to have a chance at getting a big time free agent this summer (like Joe Johnson?). We have Boobie and Telfair that would probably produce just as much as Mo did/does in the playoffs, and for less money. Or they could sign and trade Johnson to a team and get a couple solid players back.
Edit: Here's 1 scenario:
CLE in: Erick Dampier, DAL 1st round pick 2012, ATL 1st round pick 2010
CLE out: Anthony Parker, Mo Williams
ATL in: Mo Williams, Shawn Marion
ATL out: Joe Johnson (S&T max contract), 1st round pick 2010
DAL in: Joe Johnson, Anthony Parker
DAL out: Erick Dampier, 1st round pick 2012
Cavs get ripped off on talent, but get a couple draft picks and free up space to pursue a 2nd star for LeBron.
Mavs get a young, elite SG that could make the Mavs true contenders and not pretenders.
Hawks get 2 solid players in exchange for Johnson, who will probably leave anyway.