I have to say the premise of this thread is seriously flawed. There is weak evidence Ainge would have ever actually drafted Tatum at #1. Tatum may very well have been Ainge's top draft target, but that doesn't mean he would have taken him #1.
The reason is rather simple and exactly why Ainge made the trade with Philly, everyone else seemed to have Fultz as their top target. It would have been a misuse of the pick to take Tatum #1 if everyone else values Fultz more.
As for Philly, they paid a fair price to get the guy they wanted, with zero assurances he would be there at #2, let alone #3.
Both teams did the right thing for their team based on how they valued the players involved.