The best folks to write Greyhawk today work for (or own) Paizo - Erik Mona, Sean K Reynolds and Lisa Stevens. (And there's always FGG's Greg Vaughan.)
This is why I think that if Greyhawk were to be utilized it would have to be rebooted. There is virtually no way to out-Greyhawk guys like Mona unless you give up on trying to beat him at the old game.
I am not quite so foolish as to say there is no possible way that Mona and Co. could be gotten to work on a new Greyhawk, but the odds are
extremely long on that one. They already have their own highly succesful setting, one that I think evokes a feeling much closer to that of Greyhawk than the Forgotten Realms, even if it is in some ways structurally more similar to the latter.
Personally, I am an Eberron guy myself (and increasingly a Golarion guy too). But I can live with it not being a focus in the new edition. I have all of the books from the original v3.5 setting which is all I really need to keep going (and as long as Keith Baker is around the setting can never really die; I guess I do appreciate the comment about the connection between Greyhawk and Gygax).
Unfortunately, I do not think there is really anywhere to go for the Nentir Vale. It is a setting that was really tied to the conceits of 4E, and one that I think the potential of which may have been destroyed by designing its world map to work for a board game.
So I think it will come down to Forgotten Realms. Perhaps we were fooling ourselves to ever think it was not going to be the "winner" . Hell, after many years of hoping that Dungeons & Dragons Online would be expanded to include other locations, such as Sharn, we have now been treated to the revelation that the DDO expansion is going to be set in the Forgotten Realms! If that does not tell you something, I do not know what will.
I am not going to rule out the possibility that Greyhawk, rebooted or not, could be the basis for 5E. But I suspect the question in the minds of the folks of WotC is more likely to be " How are we going to present the Forgotten Realms? " rather than " Which setting are we going to favour? "