I've long argued that signing Horford without Durant was a mistake for many of these same reasons. I just believe that the cap space should have been punted another year and it would have let Ainge be more aggressive in trades for players better than Horford such as Cousins, George, or Butler. Horford is a great complimentary piece, but Boston has never had the grade A player to pair with him, which makes his money not the most effective use for that money.
I used to be really high on Demarcus Cousins, but when I saw how awful he was in New Orleans before his injury, I am kinda glad we never ended up getting him. I know we have a premium need at center and Cousins is fitting in well at GSW but I am not too heartbroken he's not on the Celtics.
As for Paul George, I am pretty sure the Celtics tried to acquire him via trade but Indy sent him to OKC instead and it turns out, Oladipo and Sabonis has turned out to be a really good package (probably better than what we could've offered.)
And Jimmy Butler's career has been somewhat of a train wreck in the last year because of his poor attitude.
So, I am glad we have Horford.
Indiana moved on because Ainge wouldn't commit until he saw how free agency went. If Horford wasn't on the books he could have committed to George and still had the room for Hayward (and I believe enough room to keep KO as well). There is also a very good chance Thomas isn't nearly as injured in the scenario where Horford isn't on the team, so even if the Irving trade never happened Boston would have still had Thomas, Bradley, Hayward, George, and KO plus Brown, Tatum, Theis, Semi, and whatever other veterans got signed. I absolutely believe that the Irving trade would have still been there using Bradley, Thomas, and the BKN pick (and if Thomas isn't as injured who knows maybe you can even keep the BKN pick and add in other future picks). The odds of Hayward getting hurt would have also been greatly diminished since it was such a freak thing and the players on the court would have been different in that scenario.
At the end of the day, you don't know how things change if you don't sign Horford, but the scenario I laid out above could have fairly easily played out and would have Boston in a much better position to win a championship as Paul George is by far the best player in either scenario and Gordon Hayward almost certainly doesn't break his leg 5 minutes into his Celtic career.