A point that hasn't been mentioned much here is him being a UFA and signing with any team he wants next year. If we kept him til that point, we'd be a better team, but there isn't anything stopping him from shopping around even if we did make him a $10m a year offer.
Just look at Radulov. He signed with Montreal for a season, had a hell of a year for himself, but jumped ship to Dallas despite Montreal offering a similar deal. Tavares, Karlsson, Carlsson, and Doughty are some other big name players who will be, or at least are currently contemplating, jumping ship in the near future despite being a big part of their respective teams locker room.
Panarin could've ran off for nothing, wanting to get out of Kane's shadow, and the Hawks would've been left with nothing to show for it (unless Panarin being on this team next season would've resulted in a cup, certainly wouldn't have been the case this year).
Maybe the Hawks get a bit more if we traded him this offseason, but that's tough to say. The player we did get in return is young and has been a fan favorite here for a number of years. Saad's bound to bounce back from his worst season in his career.