I've been on this train the moment Durant signed to GS. He is a complimentary player. He amplifies those around him, but is not someone who can be relied upon to carry the team (which is something you'd want in a max player).
Doesn't matter who is available or who's not, Horford plays the same way. Kyrie is out, 11 shot attempts, Kyrie comes back in, 11 shot attempts. He doesn't force any issues, always takes what the defense gives him. Doesn't take initiative, doesn't lead on the court.
Don't get wrong though, 10/10 times I sign him to this contract if it means we get a chance at a talent like Durant though (who reportedly wanted to play with Horford).
The market just super inflated his value. I don't hold it against Ainge, but Horford's the 3rd/4th best player on a championship team and if you can help it, you don't give those type of guys a max deal.