If he puts up 80 points, regardless of whether he is playing with McDavid or not, he is earning that contract. It puts him in the top 10 in scoring in today's NHL if he is doing that.
He has shown that he can produce without McDavid. At a slowed rate, sure, but there isn't a player in the world that wouldn't see a point decrease if he was moved from a line with a McDavid, Crosby, Matthews, or any player of that caliber. And if he does become a consistent 80 point player as a 2nd line center, he will do so with plenty of powerplay time with McDavid anyway, so I don't know why it has to be "on his own". Crosby and Malkin both point heavily on their own, but they play the powerplay together as well to help boost their numbers.
There is some projection there, I agree with that. I also think that the Oilers signed the deal knowing that the cap is going up significantly in the next few years and thus his percentage of the overall cap hit will decrease a fair amount. Agents knew that this summer, and adjusted their asking prices accordingly, unlike a summer earlier when the cap going up was less certain. Other young stars who were being paid 6 million dollars per year a couple of years ago will be getting 8-9 per year right away. The Oilers were one of the first teams who saw this jump in contract demands, but it will be happening league wide.