Pittsburgh was already protecting 7 other forwards so they were going to lose McCann for nothing in expansion, so their GM cleverly found another GM willing to trade him 21 year old former 2nd round #58 OA in 2018 in Filip Hållander and a 7th round pick, so the Leafs could then expose him and lose him for nothing instead and also pay an acquisition price to do so.
Pittsburgh got 2 future assets from McCann trade, Seattle got Jared McCann and his team leading 27 goals and 54 points and the Leafs got nothing.
The team that gets nothing and has to pay extra assets to do so in not winning in my books. IMO
Ironically the 2 Leaf players the Leafs GM was desperate to protect were Justin Holl and Alex Kerfoot, who just happen to be the 2 players mentioned and most preferred options on their way out of Toronto this summer.