I think when people look at the draftee's potential, they look at their athletic ability, basketball IQ and skills.
If the player is athletic, has good physical tools, good IQ and good skills, then the assumption is that with more experience and refining the skills the player can becomes a star.
Doncic is unfortunately not overly gifted athletically and his IQ and skills are already very advanced for his age. Now he can get in better physical shape, work on his athleticism but he is not going to start jumping our of the gym or running like a wind all of a sudden. There is only a small percentage that those attributes can be improved.
Even with the NBA training, Doncic could improve a bit athletically, his lateral movement, flexibility, muscle definition etc but the question is, will that improvement be enough given that his skills and IQ are already very advanced. This is why people are saying that he is going to be a good NBA player, a starter type but question whether he can be the franchise guy. Skills can be developed but you can't all of a sudden become an elite athlete.
People think that players like Ayton and Bagley have the highest upside in the draft and that is because they are in a way athletic freaks who run like a wind and there are no physical limitations to reaching the stardom. Now their skills have improved a great deal in the last 12 months which is why people have them flagged as potential stars. The question then becomes, what is their desire to succeed like, what is their work ethic like, what is their basketball IQ like. These are all things that are easier to teach than athletic tools. You can teach them, you can only improve on them but the scope for improvement there is pretty limited.
There has not been a perimeter franchise player in the modern game that has not be a good athlete. One thing with Doncic is that he is not overly athletic but possibly the most concerning thing is that he is not particularly flexible, in that he is not able to contour his body in traffic like the other players he is getting comparisons to (e.g. Manu, Hayward etc). He is pretty stiff in that aspect. Can he improve all of that enough to be a franchise guy? Who knows?! Time will tell but history is probably against him there. It doen't mean he will be a bust. Far from it. He is just about the safest pick in the draft. He won't bust. He will be a very good NBA player. A starter in the league and that is pretty darn good, if you have your franchise player already.
If we still had DMC and we had this #2 pick, I am picking Doncic and laughing all the way to the bank with it. The dude will not be a failure.