I think a significant motivation for the pick swap was the rookie salary scale. The 3rd pick gives us an extra $1M in cap space, and that lessens the need for some rather difficult moves to produce a full offer. I expect Ainge will be aggressive in free agency.
Even if we had drafted Fultz, bringing in Hayward was going to require other trades to rebalance the roster.
In past seasons, Ainge always had a conservative fallback path available with little negative consequences. Not this year. He has to make some changes, or the team loses out. Almost anything could happen in the next two weeks.