The slab grade of a wonder coin like this should be completely irrelevant. Unfortunately, it probably isn't. I agree that it could go for 7 figures. To the best of my knowledge, only two half cents have ever sold at auction for 7 figures, and one only made that with the juice.
When it comes to the super rare coins I don't think it makes a difference whether it's in a PCGS or NGC slab the coin stands on it's own merit. Even if this coin was in an ANACS slab it still would be the finest known. I'm of the opinion that anyone that is willing to spend that amount of money for a coin should know what they're doing. The slab is irrelevant at this level. These coins were sold way before slabbing was a "thing". Finest or rarest hasn't changed.
No, most of the whales, even when they know what the are doing, want PCGS plastic, and in an unfortunate number of cases, CAC sticky cardboard. You would be surprised at the number of Eric P. Newman coins that were sold in NGC plastic at the auctions are now PCGS graded.
There was a long discussion on PCGS v NGC on the cross over of the Eliasberg 1894-S dime and that it may be worth a bit more in a PCGS holder . It is a bit strange for sure and I think a bit of shame for special holder like the Eric P Newman coins(its part of the history of the coin to me) but that is the market for you.