Why are we talking about Doncic's defence when Magic's (even his older iterations) is never brought up in ATDs? I think Doncic's defence is perfectly fine - he's bulkier and grittier than Magic, has a more consistent motor and isn't prone to breakdowns off-ball when he's stationary. He's not going to provide significant value to a team as a defender, but he certainly won't be the weak link that drags everything down.
When Luka wins a whole bunch of championships, we can talk about him like we do Magic.
Heck, I'd just take a season where the Mavs aren't basically the same team whether Luka is on the floor or not. His on/off differential per 100 possessions for his career is just +0.2 (which is what it is this year).
Luka hasn't quite figured out how to scale back the ball dominance. He is an excellent passer, but he literally does everything, which actually harms everyone else. When he isn't on the floor, the Mavs play a much better team oriented style of basketball. They get along better and all end up playing better, especially because Luka isn't a good defender. He hasn't been able to make it work all well with the 2nd best players on his team i.e. Porzingis and Brunson, such that they couldn't wait to get out of Dallas either.
Luka is a great player and has a chance to go down as one of the handful of greatest players ever, but he has to learn how to play better within a system and not be the system, because he isn't good enough on both ends of the floor to be the system. I don't think he is even as good as Curry defensively, and Curry isn't the ball dominant offensive player, let alone guys like James and of course Jordan. That is also what sets him apart from Magic at this point, Magic wasn't the Lakers system. Magic played perfectly in the system, but the Lakers were never so totally dependent on Magic that they couldn't function with him on the bench.