There are two general reasons people think the Bulls need to dramatically re-tool. The first is understandable, they don't think Derrick Rose will be healthy enough to lead this team through the playoffs. The general consensus seems to be that D-Rose won't be D-Rose until the 2013-2014 season. However, there are enough examples of players either maintaining their level of play or improving after an ACL tear that this shouldn't be the consensus. Athletes like Corey Brewer, Jamal Crawford, Brandon Rush, Baron Davis, Kyle Lowry, and Robert Griffin the III (go ahead look those guys up, all played at a high level the following season.) The surgeon who did Donovan Mcnabb's ACL reconstruction said a year from surgery is the typical time frame for an athlete to be 100%. It's not guaranteed that Rose will be back to pre-injury form at that point, but its certainly plausible.
The 2nd reason is people don't think they had enough to win a title as currently constructed. I won't take as much time on this because I think its ludicrous. Not only did the Bulls have the best record and point differential during the NBA regular season, but they also had a team that ranked 1st in defensive efficiency and 5th in offensive efficiency, no other team had a better blend of offense and defense.On paper they were clearly the best team in the NBA this season.
As you can guess I don't think a major change is needed. I would recommend bringing back CJ Watson and Omer Asik while bringing in Delonte West and a shooter. The Bulls should be able to get West with the mini MLE( he played near the minimum this year) and while he has character concerns he was a productive player last year providing just what the Bulls need, a combo guard who can create. The shooter be it Doron Lamb, Jeff Taylor, John Jenkins, or a FA should help fill the void left by Kyle Korver. This is a team that should have its top 7 players healthy by the playoffs next year and, given how good they were this past season while overcoming adversity, I'd bet they'll stay in hunt while Deng and Rose get healthy. Another creator is whats needed, but the Bulls are too good to trade a member of their core for it.