It is a big leap to assume that if BOS keeps Brown, that they sign LeBron too. I don't think you can say for sure that LeBron picked PHI due to Brown. He might still sign there even without the trade, just based on Embiid, Maxey, and George, impossible to know, pure speculation. I would say that maybe LeBron signs with BOS if BOS got Giannis, but I don't think BOS gets him if they keep Brown. The best you can assume is maybe, and I don't see it as all that likely.
That said, it is a disappointing off season for BOS. BOS traded a good player for a lesser player to have less cap he!! the next several seasons (more optionality). I am not sure that means they dropped the ball, implying that they missed out on a golden opportunity to do something. Is keeping Brown and extending him picking up the ball and running with it? Overpaying for Giannis and assuming LeBron will sign here? What ball did they drop exactly?
Again, disappointing off season but not sure "dropped the ball" is how I would characterize it. There is going to come a time soon where you look at PHI roster/salaries and say boy, am I glad we aren't them. Probably not this season, but if Embiid crashes and burns, maybe.