It's not really a false dichotomy because with one star there's a chance we can add to the team with trades and be good reasonably quickly
Only if Rondo sticks around, and only if there are trades available in the short term for high level players who want to join a bunch of role players and Rondo.
What if Rondo is impatient with the rebuild after the Celtics end up with the seventh pick in the draft -- an 18 year old project like Gordon or Vonleh, let's say -- and Ainge is unable to make a deal for a star like Love or Melo (who let it be known that they won't re-sign in Boston if traded there)? What if he leaves for a team that isn't a few years away from competing?
Then the Celtics are left with nothing.
Any way you look at it, there is no guarantee of keeping Rondo. So you can't set it up as if it's a choice between building around him long term or exchanging him for multiple lesser pieces. We may like to think of Rondo as an inalienable part of the Celtics as long as we want him around, but that's not how it works.