This team is rebuilding - this is true.
However, in today's NBA (and even in past NBA years) it generally takes 2-3 stars on a team to genuinely compete for a title.
Wade+Lebron+Bosh Heat
Westbrook+Durant+Harden Thunder
Duncan + Parker+Ginobilli Spurs
Paul+Kevin+Ray Celtics
Kobe+Gasol+Bynum Lakers
Kobe+Shaq Lakers
Shaq+Wade Heat
Jordan+Pippen Bulls
Wallace+Hamilton+Billups Pistons
The list goes on
We already have one star in Rondo - no matter what anybody says, he is a Star player. That means as long as we keep Rondo we are at best one (at worst two) stars away from being a contender.
If we trade Rondo for cap relief we are then best case two (worst case three) stars from being a contender. Even if we use that cap space to acquire a Star, we are back where we started and still need to find one or two addittional stars.
Rondo is one of the few players in the league with a truly unique skill set - there is no other player like him in the league. Trading him makes little sense unless it's to a team that is willing to significantly overpay for him, which is unlikely Rondo is probably worth more to Bostonright now than to any other team.