There is no doubt that Lemieux is amazing and would be a star in any era. You very much underestimate how much the depth of the league has improved and how much more skill there now is.
The period between expansion and the dead puck era makes for about 90% of the top 50 point season my a player of all time.
McDavid is an outliers in that he has but up multiple top 50 seasons out of that era. To think he wouldn't skate circles around 80 era expansion defencemen, who needed training wheels on their skates, is ridiculous. Smaller, slower, less skiled players where able to excel, why wouldn't he?
Conversely, Lemieux never had to face the depth of talent that McDavid faced. He would still be a generational talent but his numbers would drop, so would Gretzky's.