All things being equal I'd rather the Celtics sign a big to replace Al Horford. But all things aren't equal, Kemba is better than any player we can realistically get in free agency. They should add the best player they can to improve the team.
This was an amazing post, TP to the OP. If we could keep Horford and sign and trade for Kemba then I think the Celtics would be a contender. Unfortunately, I think the Nets wouldn't do a sign and trade and I expect Kyrie to go there. If he wants to go to the Lakers then I think we would have a real chance at the sign and trade. This is probably the best possible outcome I can think of.