Corsi, Goals above Replacement, Points per 60, Expected Goals all show a decline.
The challenge with your presentation of Corsi, is it doesn't reflect usage. The last two seasons, he's had over 60% Dzone starts, where in the past he was closer to 50/50, and has become a match up player.
As an all round player, and as an important contributor to the team, I think his game has improved immensely over this period. I am one of the few Leafs fans, who thinks his offensive production is likely to decline this year, but I don't think that his importance to the team will. As far as trade value, it will never be higher than it is now.
You mention Points per 60... but his two highest seasons were the last two seasons... except for the lockout season.... so you are just flat out wrong there.
His faceoff percentage has improved over time, his on ice Goals against, and save percentage were very good last year.... On ice for 55 GF, while On ice for 50 GA, while starting in the Dzone, nearly 60% of the time.... that's pretty solid play, and not a decline.
His Goals Against Replacement/82 was 11.7 three years ago, dropped to 10.6 and back up to 10.9 last year. Meanwhile, he went from 51.6% Ozone starts, to 37 and 41% Ozone starts.... I mean, you can't just throw out numbers, without some perspective on them.
I think your conclusions, and stats usage are just flat out wrong. Most Leafs fans are ready to lynch me, but I'd expect him to pot between 37-43 points this year, in a more defensive role, as Tavares and Matthews get, not only the better offensive usage, but linemates. Marner was a bit part of Kadri's scoring last year, and he's unlikely to be paired with him much this year.
Kadri would be a good 2C on any team, that the Leafs have him as their 3C, is an organizational strength, not to mention the great contract.