In turn what makes you think he will be a superstar?
He doesn't look like anything special, at least yet. Even MB preferred to take Frederik Gauthier (as per post draft reviews MB was physically upset after Toronto took Gauthier, or, rather his gamble of not trading up didn't work). McCarron was slated around 45th on the pre draft pick lists and went surprisingly early most probably due to his size which was a draft goal for MB.
One can never tell for sure yet but according to analytics of the NHL draft, only 22% of players drafted in the second round will make it to play a year in the NHL, and of those 22% almost none will be a star. In McCarron's place, even though he was drafted early from his projection at 25th, his is still probably to make the NHL, but probably as an average player and not as a star.
As an example, if you look at other teams such as Toronto who has six 6'5 big hitters: Franson, Bodie, DeVane, Colborne, Fraser & McClaren and Ottawa has four: Kassian, Cowen, Gryba, and Wiercioch. Some are starters but none are supertstars. The point is: these guys are all big and big hitters but they all suffer from the same issue that McCarron does, they are not all that fast. Aside from Cowen at $3M all the rest are well below the average salary of the NHL, or in other words, getting less than average salary. Being big does not necessarily mean being good.
Nothing is 100% for sure at this point, and mistakes do happen, but these analytics are generally close to true, and that is a lot of what I am basing this on. It's only a projection at this point and look at Subban who was an exception at 43rd that turned out very well. Still though, and from what I've seen (to me at least), McCarron has not shown to being anything more than just an average big guy.