I think if you added a decent center with legitimate size who can defend the basket to this team, with a fully healthy Rondo and Sullinger playing together, this could very well be a playoff team, albeit a low seed.
With that in mind, if Ainge makes a play for Asik this summer, or if the team gets lucky and drafts an impact center (i.e. Embiid, or maybe if some guy taken later in the 1st is a surprise), I could see them making the playoffs.
Most likely, though, this summer will feature more moves getting rid of unnecessary veterans, and the team will struggle again next season. Though I do expect them to win more games next year than this year.