Doncic has played well enough coming out of the gate, no question. But there is also no question that (compared to most NBA players) he is not especially athletic nor (seemingly) particularly inclined to become more of an athlete as time goes on, through training, fitness or what have you. How much that will impact his career long term remains to be seen.
I would argue that due to that limitation, he is probably fairly close to his ceiling as to what he will be as an NBA player. That may well be good enough for him to have a fairly successful NBA career. On the other hand, as time goes on and he ages some, possibly has injury issues to deal with (likely an inevitability if his conditioning isn't optimal), and just naturally fills out more as he matures, his physical limitations may end up limiting or even hampering his career. Again, only time will tell.
By the same token, Ayton's "lack of drive" that so many seem to be eager to label him with, may or not be a real issue that likewise may or may not negatively impact his long term development, if in fact that is a real part of his makeup. The point being it is really way too early to come to any conclusion about either of these guys' long term legacy. It's annoying hearing people already fitting Doncic for a bust in Springfield (frankly, even before the draft) and labeling Ayton as a dog 25 games into each of their respective careers.