You win in the NBA with higher-end talent. Horford brings that, and he more than doubles any net loss that we lost with Turner and Sully. Simply having a two-way big man of Horford's caliber will do wonders for this team that relied primarily on the 1, 2, and 3 spots for success. I think people are really underselling how much he'll help our team this year. We'll be in the running for the best defensive team in the league, along with greatly improving our offense. Hell, just the spacing that he can provide that we didn't have last year is going to make IT's life so much easier offensively. A lineup of IT, AB, Crowder, KO, and Al will pretty much make IT unstoppable in his penetration game. Further, adding both Horford and Brown, along with Rozier getting more playing time, just makes us even more adept at the running, uptempo game that we excelled in last year.
I think the more interesting question is this - do you think we would've still got Horford if we would've beat Atlanta?
Though Durant's cowardly move to the team that took him out of the playoffs this year makes it seem more common, it's not really all that common that players, especially star players, will join the team that just knocked them out of the playoffs.