Honestly, I'm not so sure about that. He's a unique enough player (ala James Harden) to be a superstar without an ounce of defense. I think he will be the type of player that you put out there regardless of any defensive liabilities, and he's still going to be far and way a net positive on the team. He's already demonstrated he can control most aspects of the game with a high usage rate.
Maybe we are splitting hairs a little bit with the term "superstar", as it is subjective. I believe strongly that he will be a multiple time first or second team all-NBA player in his career (i.e. considered to be a top 10 player). There is of course subjectivity in everything (it's still a vote of writers to get an all-nba honor), but generally speaking I think most agree with the all-nba selections as reflective of the top players in the game in any given year.
If you define "superstar" as multiple MVPs (Lebron, Curry, maybe Harden....) then OK, I completely agree that is a transcendent, all-time great level that I'm less comfortable saying Luka will reach based on current evidence (although I would not put it out of the relm of possibility).