Except he's not a better fit for Boston than Rondo. So much of Boston's identity is keyed up in their D - and Chris Paul's D is "meh".
I think Boston would be stupid to cash in on Rondo, even for Paul. Ainge has come out and said as much too.
There's not much left in Boston's core group, and there are serious questions about Rondo as a future building block, evidenced by his lack of progression last season when he was needed to step up his overall game. There's always been a sense that Rondo's flaws on both ends of the court have been masked by Boston's schemes and other personnel. When all of that is gone, who would you rather have, Rondo or Chris Paul?
Paul will go to the Knicks, but if the Celtics had that option, they couldn't afford to hesitate.