Eh, I think that whether or not this argument holds up depends on whether you believe that, no matter how much anyone else from the 2018 draft class improves, they'll always be a step behind Doncic.
Is it, though? I'm not convinced that Doncic is that far ahead of his peers. Ahead, sure, but he doesn't seem so far ahead as to be uncatchable. 2018 wasn't like the 1997 draft, where the talent differential between the best player in the draft and, say, the fourth-best player in the draft is completely insurmountable. And it doesn't even require a so-called drop-off, like what people talk about with Tyreke Evans: Victor Oladipo won ROY in 2014, and was considered at the time to be the best player in a bad draft class. If you re-did that draft, he'd probably be 2nd or 3rd, depending on how much you value a player like Gobert. But the thing is, Oladipo never actually fell off: if anything, he started out good, and has continued to get better. It's just that he was caught and surpassed by Antetokuonmpo. Just like John Wall never really fell off: Paul George just became better than him.
What makes the pick re-litigation a worthwhile discussion to have is whether you believe that a player like Ayton, Bagley or Jackson can ever develop the skills that Doncic already has? Because one side of the argument is rooted in the belief that they can't, and the other side is rooted in the belief that, if they can, you'd rather have a guy who can do all (or most) of what Doncic can do, but is also bigger, faster and stronger. What detracts from the discussion is the tendency of some to overemphasize weaknesses to the point of making it seem like they are trivializing strengths, and the tendency of others to be more focused on making people answer for their takes.