Agree with all your points on Smart, especially regarding Brad valuing him much more than the average fan.
However, not sure how Hayward doesn't make sense to you. A very strong wing scorer who can shoot the ball and is a decent defender, who is just about to enter his prime and just had the best season of his career to date. Plus, he has a good relationship with Brad. Not sure what's not to like.
I agree with you about those points - he has a lot to offer.
I'd sum it up slightly differently: Hayward creates shots, doesn't turn it over, gets to the line at a decent clip, and makes his shots - a True Shooting % of .595 is stellar. He's a decent defender, as you say; the old NBA cliche applies: "he doesn't hurt you on defense", though he's not a stopper and not versatile. As a small-ball 4 he's a matchup problem, though you don't want him guarding bigger players for long stretches; and he's a mediocre defensive rebounder, which was a glaring weakness for Boston this past season - and he's especially poor if you play him as a big.
Here's the thing, though: Boston already has strength at the wings, and in particular a budding star in Jaylen Brown who has the makings of a big-time shot creator who gets to the line a lot - and has the tools and drive to be a stopper.
Crowder is not a star but he's the perfect complementary wing - and you HAVE to have complementary players. A Big Three and D-Leaguers is not a recipe for success, especially for long-term success. Jae's TSP% of .613 is even better than Hayward's; he is an excellent defender who can switch up or down; he's a significantly better defensive rebounder than Hayward, and he's on a great contract. Does that mean that he's better than Hayward? Good question, but it's true that Hayward has the rare ability to get the team shots, and that rare talent is absolutely essential in modern basketball.
I would agree that Boston needs a star, a shot creator, but they're better off getting a big who can create shots AND improve their defensive rebounding.