If McDavid retired today I might have a hard time placing his *career* ove the likes of Sakic, Yzerman, Kane, and Malkin given that they all played extremely important roles in multiple Stanley cups to go along with the longevity and counting stats + individual awards etc.
I would say it is all but assured he passes that group, even without a single cup assuming he can play long enough to accumulate 1500+ pts (that's actually quite a low-end projection but anything can happen). I would say he is certainly not above the Messier/Crosby/Lidstrom/Ovechkin/Jagr tier and likely would need at least 1 cup to pass those guys given their individual and team dominance + longevity. Without a single cup he is probably going to be somewhere in that category no matter what he does individually, at least in "public opinion".
As it stands he's not top 10. Going off just guys who have played since the 90s he is below Gretzky, Lemieux, Crosby, Ovechkin, Messier, Yzerman, Sakic, Jagr Malkin, and Kane. Obviously they are before my time but I don't think anyone is ranking him over Howe, Orr, Lafleur, Beliveau, Hull, Richard etc. That's 16 guys I can say for sure right now in terms of career and all time ranking he is below.
Personally I think he will likely pass Kane , Sakic, Yzerman, and Malkin so long as he can just hang around, even if his play drops off a level. Catching Crosby/Ovi/Messier/Lidstrom/Jagr is not guaranteed but also very possible. All in all I would say there's about an 80% chance he ends up top 10 all time with the possibility to even get into the top 5 because the talent is there.