It certainly isn't the case that Baby "must" stay. Even if he lives up to to highest expectations, he'll be no more than a good NBA player. And that's certainly something that can be replaced.
I also worry about people's perceptions of him. Sure he looks good on offense. But couldn't a lot of players look good playing with Rondo and the Big Three. Think about how successful Posey was. Or how much better Shaq looks now than he did last year.
I'm not sure Baby looks this good if it's him, Rondo, Perk, and two no-names.
The problem is, no team with Rondo on it is ever going to be bad enough to win the draft lottery. Thus, in all likelihood we're going to have to rebuild by signing at least 1 (if not 2) big time free agents. Signing Baby to a sizable extension could prohibit that. And if that's the case, he's not worth retaining. He'll never be good enough to be the #2 or even #3 guy on a title team.
The only exception may be if Ainge thinks signing Davis could enable us to use him as a trade chip later.