Dallas lets Luka have rebounds? That's a bogus argument, how about Luka commands the area more and collects the rebounds. The fact that you argue that Harden has to share the assists with Westbrook shows Luka's importance to the Mavs more, Luka has to be the prime scorer and playmaker, something neither Harden or LeBron have responsibility over.
Harden averages 38 points while Luka averages almost a 30 point triple double a game! Westbrook won the MVP over a similar 30 point triple double season (though in a less efficient way).
And last I remember the Mavs were 2-3 without Luka, albeit in a really rough stretch of games. Though they have also already beaten the Lakers and Rockets away, Raptors at home, with Luka as well.
Luka does more on the court than Harden, he's the playmaker and scorer for team that's doing well above expectation