Let's be clear, here: Butler was never an option for the Sixers, he didn't want to be there. And in truth, he wasn't a great fit there anyway. Butler and Harris were completely redundant and one of them was not going to come back. Butler made that choice for them. He wanted to go somewhere he could be the unquestioned alpha. In Philly that is Embiid. Butler served as the closer and guy who was there to take over offensively when things got messy. Otherwise, he drifted through games. That was not a role he desired. Harris seems to be fine with that role, even if he isn't as good at it.
That aside, Horford was a smart but admittedly risky bet. He is being asked to fill a very unique role, the value of which has been set, but yet to be determined if it was justified entirely. Early returns are that Horford is giving Philly exactly what they needed from him, as he stands a very good chance at being the difference between something like a 3-4 seed and being the top seed. I don't think Horford was the only option Philly had, as they certainly could have received a trade exception for Butler instead of the ability to sign Horford, but let's not forget the Sixers also received Josh Richardson. Richardson is a pretty solid piece himself and certainly was worth some price there.