Yeah, Luka has a ceiling due to defense. Note that I actually don't think he is a bad defender, but almost all "top-whatever number we pick" are actually good on both ends of the floor. This includes James, Duran, Leonard, Davis, Giannis, etc.
That said, there are guys like Curry and Harden who are just so good on offense that their defense is overlooked. Again, Doncic is not as bad as those two, so I am not making a comparison here.
There are so many great players in the league, so I am not sure if I'd predict top-five or top-ten, or whatever else for Luka, but he sure as hell looks like a top-30 in his rookie season. 20 points, 6.7 rebounds, five assists, 1.1 steals, 37% from three...in his rookie season while clearly being the team's best player and being guarded by the best defender night in and night out. Again, I don't know if he'll be top-five, or top-ten, or top-whatever, but I am not putting a ceiling on this kid.
In January, he is at 23.3 points, 7.9 rebounds, and 5.1 assists, with 37% from three. In November, he was at 17.7, 6.7, and 4.2 on the same three point shooting percentage. That's 5.6 points, 2.2 rebounds, and almost one assist improvement from November to January. Yeah, definitely not putting a ceiling on him.