The "Winning a Cup" argument will become less & less relevant to determine a player's greatness as the sport expands to more teams.
It used to be a real method to determine a player's greatness, to an extent, when there were 6 to 12 teams. But in modern day sports, where the leagues have made it harder for players to get paid what they're actually worth for teams to build sustainable teams with depth that can last for a long time, winning is incredibly hard.
Better metric should be, has McDavid performed up to his talent at the key times? And the answer to that, in my opinion, has unequivocally been yes.