Honestly, this seemed ridiculous at first, but the more I think about it the more it probably works for both teams. Pens clear major cap space, and acquiring Tavares keeps them a contender to repeat.
Isles lose their franchise guy, but with the way things are going in Brooklyn, they might be losing him anyway. At least now they'd be getting back another superstar who is better than Tavares anyway when healthy, as well as a star goalie. Their on-ice product would improve. Prob not a great long-term deal for the Isles though.