Keep in mind that Smart should be able to play some minutes at SF as well, next to Rondo and Bradley.
With such a deep draft just taking place, the market for Bradley might not be as hot as you think, and we could use more than one defensive stalwart in the backcourt rotation, not to mention Bradley's outside shooting.
Plus, why would we pay Rondo the max or close to it when we already have Smart.
Letting Bradley go might save the Celtics 4-5 million or so, but trading Rondo would save the Celtics much much more.