When Leon signed for 8.5, he was maybe considered like the 8th-10th best player in the game in 2017, now a lot of people put him in the top 2-5 range, he's won a Hart trophy, a Rocket, multiple 50 goal seasons, etc. etc.. McDavid was the best player back then and he is now, so that hasn't changed much.
McDavid also makes a good chunk of money from endorsements, $5+ million extra per year which has been going on now for a long time. That's a luxury only he and Crosby and maybe to a lesser degree Matthews have.
A generational talent, especially a Canadian born one, like McDavid always has tremendous pressure to win a Cup. So I don't see him taking a contract that will negate himself from winning, it's not a little "nice to have" extra to win for him, he has to win.