I'd be curious to know who you think is an "equivalent" player to Alex Kerfoot, that makes $2m, that wouldn't cost an arm and a leg to acquire?
Going one step further... let's say the Leafs went 7F, and protected both McCann, Kerfoot, and Engvall.
You're now in a position where you've got both Kerfoot & McCann on your left wing, which probably isn't ideal being that they're similar players, or having to put one at 3rd line C, which basically prevents you from building the shutdown line that the Leafs have around David Kampf.
It also means that unless you go out and subsequently trade Kerfoot, you cannot sign Nick Ritchie to improve this team's toughness & size.
Assuming you still sign Kampf, you're then shopping in UFA to replace Holl with with $1.5m (as McCann is ~$450k more expensive than Ritchie).
If you trade Kerfoot and sign Ritchie, yes, you can go and try to replace Holl with about $2.5m in cap space to do so, but still, not a lot of Holl-replacements out there for that kind of money. I struggle to think of any, to be honest.
Bottom line... you want to argue that the Leafs would have been better off with McCann over Kerfoot? I don't think many will argue with you, they probably would have been, especially if Tavares/Matthews are able to remain healthy all year, and the $550k in cap space is not insignificant.
But, the difference really isn't all that big, and certainly not as big as what the Leafs could have reasonably projected to be between Justin Holl and his replacement.