While Boston the city is never going to be a hot destination for free agents, I don't think it'd be impossible to attract a marquee guy because stars will go anywhere (within reason) where there are other stars. The problem is the talent in Boston isn't good enough to attract anybody right now. Rondo is the closest thing we have to a star but he's obviously not in that top echelon.
When Ainge acquired the Wallace deal it was clear this was going to be a two year rebuild at least. That's why the Celtics should just continue to build assets for the time being. Getting some lottery picks is part of that strategy.
Even if we somehow got a disgruntled "star" like melo to Boston, that's not enough to be a contender. You'd need another third star who can score, especially since Rondo will never average over 15 ppg. You'd also still need a big man who can play defense. The Celtics don't have enough assets right now to acquire so many pieces, nor will they have the cap space to sign somebody the old fashioned way.