Most likely, it is a combination of bad talent evaluation, poor draft trade, misguided roster construction and a failure to develop Fultz's talent so far.
Fultz or BC will likely get most of the blame, but everyone else that was involved should not escape without accountability. As far as we know, the owners signed off on it, Brown approved it and the other folks like Cohen helping the GM approved it.
They may start to distance themselves from this decision if things continue to go south, but I doubt any of them were that opposed. Fultz was widely accepted as a top pick (maybe not THE top pick) and certainly had shown in college the range of skills to play at the NBA level.
People moan when Okafor and Noel come up again, but this is a historical pattern in this 76ers era. If Fultz was the first situation like this, one could shrug it off as an outlier, but he is not unfortunately.