While the Toronto Maple Leafs could use another top four defenseman, doing so is not a make or break for the Leafs winning the Stanley Cup.
Look at what the Pittsburgh Penguins were able to do with an even worse defense than the Leafs have. Their typical top six was Justin Schultz, Ian Cole, Ron Hainsey, Olli Maatta, Brian Dumoulin and Trevor Daley. Mark Streit played for three games and Chad Ruhwedel played for six games.
I do not think you can even try to argue that their defense is better than what the Leafs have. Now, of course having a strong defense is important, and just because a team got the breaks necessary to win without a strong defense doesn’t mean that that is a recipe for roster construction.