I'd happily trade IT for Hayward (I also agree that Utah wouldn't do it).
Hayward is a more well rounded player, could probably fit into a significant role on pretty much any sort of team, and while he's only a year or so younger than IT, he will probably be able to play at a high level much longer.
Trading IT at some point before his contract is up will most likely make a lot of sense, provided the trade is for a high quality piece that could be here a while. The Celts probably won't be in a position to contend before IT's next contract, and his next contract will probably last beyond the point that he's an above average player.