That was peak Dionne vs rookie Gretzky, not a great example. Nicholls was quite obviously a product of Gretzky and either way, 18pts isn't exactly close at all.
That's the point, he would clearly be lagging behind. Lemieux is a bit hard to say because of injuries, but over the 80s decade (80/81-89/90), Gretzky AVERAGED 185pts a season. Give Crosby 150. Hell, let's be SUPER generous and give him 160. That's still 25pts per season. In other words, Crosby would be miles behind.
If Crosby was on the pens at the same time as Lemieux then his legacy would take an even bigger hit. Think about Crosby and Malkin. Crosby is the face of the franchise, teams gameplan for him, he's the captain, etc. Malkin gets disrespected so often in the media and his legacy is looked at as "meh" because he's usually seen as the Robin to batman. And this brush off of his legacy happens despite Malkin having the same number of rosses, a better peak season, a better peak playoffs, etc. Now imagine Crosby on Lemieux's team. He's going to be the sidekick like Malkin, but also a clearly inferior player. He will have zero argument as a top 5 player in history.