If you are rebuilding you don't hang onto a player that won't let you be truly awful. It is far better to be downright awful then to be mediocre.
You are better off barely missing the playoffs and hoping to get lucky in the lottery than to be downright awful. To be truly horrible, you either need to be destroyed by injuries, collapse after a star leaves your team, or have a mediocre team and pick busts in the lottery multiple years in a row. Or maybe hire a moron as your coach.
The Celtics have pieces to be contenders through 2014-2015 (or at least through next season). If the Celtics can get two rotation-caliber players from Fab Melo and their next two years of drafts, the Celtics have an excellent cap situation with a relatively young (and cheap) core that would allow them to re-sign Rondo in the summer of 2015 and still possibly have room for a max free agent.
Of course, if Sullinger develops into a Paul Millsap-level PF and Avery Bradley improves so that no one sneers at the idea that Joe Dumars is his ceiling, and Fab Melo follows the same career trajectory as Ian Mahinmi, and Jeff Green plays well enough so that the consensus is that he is an adequate replacement for Paul Pierce in the starting lineup, then maybe the team doesn't need a superstar free agent to be a contender. Everything working out perfectly like that is unlikely, but it's reasonable enough that you're not crazy for having some hope that it can happen.