There is one big aspect to this type of a deal that everyone is ignoring imo. That is, it is generally a very bad practice to trade within your division, especially to your long term rival who is also at a similar stage in rebuilding as you.
I mean, let's pretend for the sake of argument that we all want to trade #3 for Okafor for the benefit of the Celtics. By doing so, we give the Sixers exactly what they need to balance out a stacked roster of top tier prospects, giving them a core of: Embiid, Noel, Saric, Simmons, and Dunn. We would be likely facing that team in the playoffs for a decade. The Dunn trade could haunt us for an entire era.
If we don't make this trade, they are forced to go ahead with no good PG, and three young top prospect centers, with the whole league knowing they need to move one of the 3 , and lowballing them.