I don't necessarily think it was the wrong decision to hold back on making a deal at mid-season. I think what we're seeing in the playoffs right now is proof that this team was not one mid-season acquisition away from making a deep run.
I was very surprised when Horford signed with us last year. I didn't expect it. At the same time, I don't think Horford expected that the Hawks would balk at giving him a full max deal.
This is speculation but I think you're mis-reading what happened. I think Horford was nothing more than a lure for Durant. He was Ray Allen 2.0.
At that point, Danny thought we had a really good shot at KD. And we probably did.
But, the chips didn't fall our way, and my personal guess is that since the day Durant signed with the Warriors, Danny has viewed this as a dead team walking for the entire year. That's why he didn't invest in acquiring role players (we have no chance even with them) or stars (even Cousins wouldn't push us over the top).
Stevens once again made life difficult by shoe-horning us into the #1 seed and the curse of expectations. But that's no one's fault. Nor is what's happening now, which is an outcome that would be unsurprising if we were the #4 seed (which, but for a few injuries and other things, we could be).
I think Danny has his mind on May 16th and beyond, and he has no regrets about the decisions he made this year, because he believes we never had a real chance.