In retrospect, the reality is that Hayward had a perfect situation in Utah and it was unlikely that he could find a fit like that anywhere else. That's not necessarily a knock on him, to be fair. Unless you're a top player in the league like LeBron, Durant, Harden, or Leonard, you can't just have random players put around you and still succeed. Those lower tier All-Stars need specific situations and players around them in order to do well, and once you remove those, the whole thing falls apart. When you look at it objectively, there may have been only one or two other teams in the league where Hayward could put up 20-5-4 as a first or second option and win 50+ games and be an All-Star.
The problem is that Hayward, like many lower-tier stars, didn't realize this. Even if he didn't get hurt, even if he was the same player right now as he was a year and a half ago, chances are that he wouldn't be putting up 20-5-4 or be the second most important player on that team. Horford and Kyrie would probably still be ahead, and possibly Tatum. He's just not that good of a player that he can go to any team and instantly be a star there as well. He needed a specific situation to be a star in SLC, and he needed one in Boston. Boston wasn't the situation to begin with, and it's even less so after the injury.