Thing is, it is easier to prove Kessel is among the very elite at his position than it is for Phaneuf. you can certainly make a case that Dion is a top 10 D, but it's a easily arguable point. There is no chance whatsoever you could call him a top 5 D. Meanwhile Kessel is top 5 at his position...that's a guy you give a max contract no questions asked.
I do think that this season could be Dion's best yet for the Leafs, as with Gardiner and Ranger getting full time spots the talent around him on D is better than when he arrived and better than last season. We're not going to be playing him with bottom-pair or borderline AHL players like Aulie, Kostka, Holzer etc etc this season.
and yes, it's highly doubtful we'd get enough value back for him in a trade -- the Flames got nothing for a younger Dion with more years under contract -- and we'd be left looking for the #1 top D again. we'd probably get a player, 1st round pick and prospect. that's not terribly enticing given the hole you'd be creating.