I agree - a very good analysis, and pretty realistic I think.
I do believe that adding Horford (and bringing back the rest of last year's crew, sans Turner) on it's own moves us forward a significant margin. Certainly not enough to be considered contenders, but enough to the point where I think we would be a lock in favourites to finish 2nd in the East.
As they say, adding Durant pushes us well beyond that.
In the short term there really is no negative tot he Horford signing - there is no possible way that it makes us anything but better. In the long term there are risks, but if Durant joins us that all goes away.