I miss Horford, but honestly at 4/109M I think it was best he left. Remember, we signed Horford at around age 29 to a 4/112M max deal at the time. And now Horford's 33 years old and basically got another "max" deal.
This gets us under the luxury tax (to reset it and not have to worry about the dreaded "repeater tax" anytime soon), and in Ainge's/Wyc's defense, even they couldn't have known this team would be THAT good, to the point where he felt it could be worth paying a luxury tax for the 3rd straight season.
And anyways, as others have mentioned, it did seem the C's were open to bringing him back, but probably something around 4/80M. Not what PHI gave him.