It's not a bad argument at all..you just need to fully grasp the concept of projection.
There are things that stop a player from being an elite player, and to loosely group them:
Character Issues - receptiveness to coaching, effort level, dedication, leadership
Lack of Skill - shooting, puck skills, passing, faceoffs, defensive stickwork
Poor Athlete/Genetics - skating (speed and agility), strength, balance, coordination, height, weight
Lack of Intelligence - problem solving, reasoning, positioning, anticipating
We know that Rossi has a brilliant character, possibly the best in the class. We know that while he isn't at the top of the class, he doesn't lack skill. He's a very smart player. What we don't know is whether or not his genetics and athletic ability will hinder him when going up against the best of the best athletes and players in the world. And when we need to step back and assess whether he can improve that area to the point where we know he will be able to excel at a high level.
Take character consideration out of it. We all would love Marco Rossi for the fact that he would walk on hot coals and crushed glass if someone told him it would make him better. That being said, what people are discussing render his habits and his mindset completely irrelevant. We know he will work hard, but some people just flat out can't improve the athletic side of things enough to overcome certain limitations. The kid will play in the NHL, maybe even in 2020-21, but some people are just weighing upside vs safety, and upside is winning.