I think another thing to consider is the luxury tax. If we'd bought back Horford, we'd be over for a 2nd straight year, and in Ainge's defense I don't think we knew for sure how this team would look with Kemba and much of the same (minus Kyrie, Rozier and Morris). As a result, to avoid paying a repeater tax in a few years for potentially an "average" group, they decided to stay under and it affected what they were willing to offer Horford (if they were even considering it).