I disagree with your undervaluing the difference in game changing skills from Sully to Horford. You haven't proven it or even tried. Horford is an all-star and was the second biggest free agent. Only Toronto seems to have wanted Sullinger and for a small price compared to Horford. I do not understand your point. Sully was terrible or serviceable at best not too much better than a Tyler Zeller. I'd rather have Sully than David Lee, but that's not saying much. You seem to be underrating Horford.
The highlighted part isn't "skills", it's reputation. The point I am trying to make is that precisely in terms of skills and production, the actual
marginal upgrade over Sullinger here is not as big as it looks when you just say "oh, we added this allstar to the roster".
Horford is a skilled all-around player and a great guy to have on your team, but he's not a go-to scorer, and not a great rebounder. He's more efficient than Sullinger offensively, and will likely soak up more minutes. The last aspect is not negligible, because it means that while Sullinger was in the rotation, there were 10 extra minutes going to the likes of Tyler Zeller and Jonas Jerebko.
Also, I'm sorry, if you seriously claim that Sullinger was "not much better than Tyler Zeller", I will have to respectfully question your judgement and understanding of the game of basketball.
The Turner loss is addition by subtraction. He is a ball hog (although he seemed to do less of that) who would have cut into Brown, Rozier and Smart minutes. Evan isn't better than Jae Crowder and isn't known for being a good small forward. He is a point-forward?
Turner was about the only player on the second unit that seemed to be able to handle the ball without dribbling it off his food or passing to the third row. He wasn't a ball hog, he was asked to handle the ball and run the team a lot because Marcus Smart was an unmitigated disaster and no-one on the second unit had any offensive capability other than hitting an open jumper.
There is still no guarantee that the team has adequately replaced this aspect of his game. Right now, we figure to be leaning heavily on a second-year player whose only credentials so far include tearing up the preseason and summer league.