I want to say upfront that I'm not by any means implying that Thompson is right now as good a player as Horford. As a matter of fact I love Horfords game and he's a much better low post defender than Thompson and has a better low post offense. He's a solid player.
Horford: 14.2 PPG - 9.9 RPG - 2.3 APG - 1.1 BPG
Thompson: 12.5 PPG - 8.5 RPG - 1.7 APG - 1.0 BPG
As you can see, they're not that far apart stat wise, and Horford just finished his third year while Thompson just finished his second. Now granted, Horford is the better player. But how much are you gaining and look what your giving up. Your giving up a chance at a potential superstar at best and possible all star at worse. Not to mention that Horford is really a true PF at just under 6'10" in shoes.
I just don't see where we gain that much, and I really doubt that Atlanta would deal Horford anyway. They would almost have to draft either Favors or Cousins. If they choose Favors, they might as well just stay with Horford. And despite what I might think about Cousins, there's no guarantee he would be better than Horford. So from their point of view, why do it? The only way they might gain is to draft either Wall or Turner, and then what do they do for a center?
Now if you want to trade Horford straight up for Thompson, then were talking.. Somehow I don't think they'd do that either.