Ah, the AS and conference finals appearances are pretty much the ONLY reason he decided to move to your inferior team in a vastly inferior conference (also the bromance with Stevens maybe? Seems silly). I doubt that the fact the Celtics are developing a 3rd pick who plays the same position played any role in the decision. Or the fact that the Celtics have more potential to acquire players years down the line, when Hayward is in his 30s.
He wanted an easy path forward full of accolades. I hope he chokes on them.
Either way, Hayward won't be enough to put the Celtics over the hump. The smart thing would have been to trade IT and rebuild using the plethora of picks and young players BOS has. They'll probably still do that once they realize they aren't much farther along with Hayward than without.