Horford is a 3rd tier all-star as is Isaiah.
Superstars Tier 1: LBJ, Durant, Curry etc. (Prime KG was here)
Perennial All-stars Tier 2: Lillard, Griffin, Aldridge, Melo etc (Prime Pierce and Allen were here)
Fringe all-stars Tier 3: Derozan, Horford, M.Gasol, D.Green, IT etc (Rondo was/is here)
I pay the top 2 tiers max money no question, but tier 3 I'm not as willing, especially considering age. Paying fringe all-stars max money doesn't seem like a recipe for success. Imo, the signing only made sense with Durant.