So the Sixers gave up Maledon and a protected 2025 first along with Horford to get essentially Danny Green.
I still don't fully understand what the problem was for Horford in Philly. I can understand OKC not wanting to take minutes away from young players and give them to a guy who might actually help them win some games. But why couldn't Philly figure out how to get something out of Horford? Even if all they wanted was defense, rebounding, and passing. Al doesn't need the offense run though him or anything like that. The Celtics aren't "running through" Al Horford. This seems like a coaching failure on Doc.
Philadelphia 76ers President of Basketball Operations Daryl Morey announced today that the team has acquired Danny Green, and Vincent Poirier (pour-ee-ay) from the Oklahoma City Thunder in exchange for Al Horford, a 2025 protected first-round pick and the draft rights to Théo Maledon (No. 34 selection in 2020 NBA Draft) and Vasilije Micić (No. 52 selection in 2014 NBA Draft).
They didn't think Al had lived up to the value of his contract and felt like they could use the money elsewhere. And they probably weren't wrong given, they got very similar production from Dwight Howard at 2.5 million the next year, added Curry as well, and had Green all for less than they paid Horford. I think it was quite simply them trying to better use the money i.e. 3 rotation level players (Green, Curry, Dwight) instead of 1 (Horford). Now maybe they still could have had Curry (though maybe not), but then you are significantly increasing your dollars. Dwight wouldn't have made sense with Al on the team so he wouldn't have gone there.
All the payroll considerations that you list are true, I am sure they are all things they considered, but only after it was perceived that it was not working with Al Horford on the Sixers. They weren't getting much out of Al Horford so then they figured they could make better use of the cap space. But it was a coaching failure in the first place apparently in that they were getting so little out of Al, that trading him for Danny Green and signing Dwight Howard seemed like it was better than keeping Al. Think the Celtics would trade Al Horford and throw in a first round pick for Danny Green and Dwight Howard right now?
Al was/is a bad contract, that is not in dispute, but Al is a much better player than what he was on Philly, or at least is contributing more to our winning than he was perceived to be in Philly. Something went wrong in Philly as it usually does. They should have been able to get more out of Al than they did. This season for Boston, he put up 10 pts and 8 rebs and that contributed to historically good winning basketball. His last season in Philly (well only season I guess), he was 12 pts, 7 rebs, 35% from 3 and he was a problem that needed to be shipped out with a first round pick.