to be fair, Luka is way ahead of where Harden was at his age. That dosent mean he would continue evolving, but its hard to think at this point that he wont improve. He is only 21. This is the Luka Doncic era guys.
I'm not saying he will turn into 22 old LeBron physically. For him, it's enough to improve a little athletically each year and he will be fine. There's a stark difference in his first step from his first season to second one, but it's hard to see it if you don't pay close attention or you don't watch enough games. He is also much better at finishing with his left arm if you compare him to his ROTY season.
Harden was a 6th man at that age. Luka is the first option on the Mavs. Very different development trajectories. IMO, I think OKC should have started Harden from the beginning. If they did, they wouldn't have traded him away and could have won a championship before GSW's dominance began.
Maybe he just lost weight and worked on his strength and conditoning. I dunno how you become better athletically I'm sure if there was a way to be athletic Shane Battier, Steve Nash, John Stockton and Trae Young would be all over that.