This is a very good trade for the Leafs, acquiring a top 4 defenceman who will likely play next to Norris trophy candidate Morgan Rielly.
Jake Muzzin is signed through next year & this season will only cost the Leafs 1,483,871 in cap space.
The players the Leafs gave up are not currently on their roster and who knows if they ever would have played for the Leafs. As for the first-round pick they gave up he would have been a low first round pick, which often fail to find regular NHL employment.
It's a very good trade for both teams & Muzzin likely would have ended up costing more had the Leafs waited until the trade deadline to acquire him. And had this happened some other team would have likely scooped him up.