Second, I love Crowder, but he's overrated right now. Brad Stevens is one of the best offensive coaches in the league. Quin is not. So these guys numbers are a bit inflated IMO, kinda like how role players under Jerry always regressed under other teams.
Third, IT is a good player, in his role. He's a 6th man, scoring option. But he doesn't play defense. In rare circumstances you can do alright with that kind of player, like Boston did, but they just did alright. Never will you have a contending team with a player like that unless you have a LeBron, Durant, type of player around him.
In short, we're overvaluing Boston players because of Brad Stevens, and undervaluing Gordon Hayward, who is really freaking good, because Quin is an awful offensive coach so far.