Yup. I think this team already is a contender. Horford is an upgrade over Bass in nearly every aspect of the game, especially rebounding.
As for the future, I think Rondo is a better player than Horford. But point guards are much easier to come by than big men in the NBA. And when point guards do prove they are NBA caliber, they can be had on reasonable contracts (Dragic, Lowry, Rondo, Conley, etc.). When big men even just show potential to be starting NBA caliber, they instantly command 5 year $50M contracts (Mcgee, Jordan, Hibbert, etc.)