Some teams in the league today are a big risk away from being able to win a cup.
Leafs have been top 3 in the division for a few years now - but let's be honest, top 3 is really more #3. And this year - they're slipping right now, it's not even for sure they make the playoffs yet. So as good of a core and base that they have - actually going all the way through to winning a cup may not seem that close.
PK is a big risk, a big gamble. If he tanks - his huge salary is a problem. He's also had history of not always getting along with teammates/coaches or causing locker room issues (which I personally think is BS - but we've all heard the stories), which could also be a concern.
Flipside is. Even in his down year in Nashville - he was their best player in the playoffs last year. He's been their best player in pretty much every playoff run, including the cup finals run (him and Rinne). He was great in every post season with Montreal. He's also an extremely passionate, proud player. Playing in his hometown, and a hockey market as huge as Toronto, would likely be a huge motivator for him.
It's a big gamble. But I could see one version of this story where Leafs acquire PK, and together they go on to win a cup. I doubt Leafs are willing to take on such a gamble though.