So Dion is sharing shots and minutes with two other All-Star players? Just look at the usage numbers of both guys in years one and two.
Even so, Harden's advanced stats crush Dion's. He was also a year younger. In year two, his TS% is 10% higher and his EFG 5%. That's not even taking in to account the massive jump he made in year 3, after already being WAY ahead of Dion at a younger age.
Is DeMarr DeRozan even worth comparing? They're two completely different players. He's also 6'7" and a better athlete. If Dion possessed his non teachables, I'd be far more patient with him. Instead he's an undersized 2 guard who can't shoot or finish.
DeRozan was a far better finisher early in his career (60+). He also was markedly better at getting to the line. That's Dion's biggest issue at this point.... he sucks at finishing at the rim and he seemingly can't draw fouls. Given he's a sub par jump shooter, it causes his efficiency numbers to absolutely plummet. Unless he's going to grow or become a better athlete as he ages, I don't see that marker moving at all.
I think Dion can get better but the guy is far more limited than anyone ever lead you to believe around here.