Saying he will average 2 PPG eventually is completely baseless. He's likely in his peak season right now, and isn't at 2 PPG.
Give McDavid his best case scenario this year (60 goals/150 points with second place being at 115).
Gretzky vs McDavid after year 8
Point finishes
Gretzky: 1,1,1,1,1,1,1,1
McDavid: 1,1,1,1,1,2,2
Remove same finishes
Gretzky: 1,1,1
McDavid: 2,2
Hart
Gretzky: 1,1,1,1,1,1,1,1
McDavid: 1,1,1,2,3,5,5
Removing same finishes
Gretzky: 1,1,1,1,1
McDavid: 2,3,5,5
VsX
Gretzky: 145.0
McDavid: 115.4
Goals
Gretzky: 1,1,1,1,1,4,5,7
McDavid: 1,2,6,7,7,10
Assists
Gretzky: 1,1,1,1,1,1,1,1
McDavid: 1,1,1,2,2,2,3
Leading playoffs in scoring
Gretzky: 4
McDavid: 1
Cups
Gretzky: 3 (1 Smythe)
McDavid: 1
The gap between the two after year 8 is massive. Like unbelievably massive. Peak season, best 3, best 5. Best playoffs, total resume. It is not fair to McDavid.
After year 8 Gretzky still had
Point finishes of
1,1,1,2,2,3,3,4
2x leading playoffs in points
Cup (and Smythe)
Hart: 1,2,3,4,5
This isn't even getting into Gretzky's international resume, the best of any non Soviet, since McDavid hasn't had the chance to play.
McDavid is too far behind Gretzky to ever catch him. If that's your expectation you're setting yourself up for disappointment.