Ok, but there's a major flaw in this argument. You don't have 2 Scheifele's so you can't somehow equate 1 Scheifele to having 2. Yes you get the extra cap space, but you don't fill that with a player as good as Scheifele, you fill that space with players like Perreault and Myers instead of players like Benning or Caggiula. Yes, that adds value, but think of it this way. McDavid could play with 2 players making under 2M and still outproduce Scheifele, aka create more goals. Now, those players would likely not be contributing on your power play, but there's only so much PP time to go around to your highly paid players so you're not wasting PP scoring by having McDavid at an extra 6M. You are however able to score more goals at even strength without having to have guys like Wheeler and Connor on the wing who will easily be making 14M combined on their next deals.
Its nice to think of it as simple as 2 Scheifeles vs 1 McDavid, but that doesn't actually exist in reality. Having a player who can be involved in 100+ goals in a season without having to feed him highly skilled wingers is quite easily worth the extra dollars.
Additionally, even if you want to put a higher paid winger on McDavid's line which is sometimes the case, you're still saving that extra 5 or 6M by being able to throw a cheap guy on the other wing like they have for the entirety of McDavid's NHL career.