If you trade Horford now no Free Agent will ever come to Boston again. You simply can't make a big splash to sign one of the best FAs, talk about how he's an ideal fit for the team and dump him after half a season because you're chasing something better. It's an absolute nonstarter. If there's one guy on this team who doesn't have to worry about being traded this year unless he wants to be traded it's Horford.
It is not like you're exiling Horford to a lousy team or trading him for some equivalent player. Horford would be going to a championship contender. Durant >>>> Horford. The trade would be a no brainer assuming Ainge felt there was a good chance to re-sign KD.
You don't get it.
It doesn't matter if we're "exiling" Horford. During the FA process the Celtics Front Office made representations to the player - how'd they get used, how much they'd play, how they fit the system, how much they'd be desired, etc. Beyond cold, hard cash that's how players make decisions on where they're going.
It'd be one thing if Horford was the 15th man on the bench. Teams, agents and players know that those guys are a dime a dozen and nothing more than cannon fodder. But for a big name like him - one of the gems of this last FA period - you simply can't dish him off a week or a month into the season. No free agent or their agent would ever trust the Front Office again. No one would ever sign in Boston again as long as Danny was in charge of the Celtics. It's the equivalent of cutting your own throat.
As such, it'd never happen unless Al himself pushed for the trade. That would be the only saving grace. I can tell you with assurity that the chances of Durant in the FA market after f*king Horford over would be absolutely nil.
It ain't a video game, kids. In the real world there are consequences to being a bad actor.