The verdict
From a Maple Leafs perspective, it makes sense to trade the salaries since they're close to even. Liljegren has two years left on his current deal and is a $3 million cap hit for both seasons. As for Marner, he's in the last year of his current contract, where he's a $10.9 million hit. On the surface, it sounds nice.
However, the Golden Knights must think about other ramifications of the deal. Is it worth having two years' worth of Liljegren before he becomes an unrestricted free agent on top of a year of Marner? That's especially true when he only has one assist in 13 playoff games (Pietrangelo has 13 goals and 61 assists in 139 games). Yes, sample size is a factor here. However, Pietrangelo does have a no-move clause (like Marner), which complicates things.
Still, that's not to say they shouldn't make the deal for Mitch Marner. After all, McCrimmon doesn't mind getting younger pieces who will make the team better. However, it should be reworked to fit them better in the long term. The current proposal could be a short-term move that ultimately hurts Vegas in the long run. That's something no team can afford during their Stanley Cup run.