I entered the hobby in 1989, right at the start of AD&D 2nd edition. That's a long time in the hobby, but not long enough to have been there for the heyday of Greyhawk. I always considered Forgotten Realms the vanilla fantasy setting of D&D, whilst the other settings (Planescape, Eberron, Dark Sun, Ravenloft, etc.) gave a fundamentally different play experience.
From what I've read, Greyhawk is just vanilla fantasy with a slightly different tone than Forgotten Realms. (And tone is easy enough to establish by the DM and the type of game he or she runs.) So my take is that Forgotten Realms has supplanted Greyhawk. I can't see GH as anything other than a superfluous, vestigial setting (with Blackmoor and Mystara).
From what I've read, Greyhawk is just vanilla fantasy with a slightly different tone than Forgotten Realms. (And tone is easy enough to establish by the DM and the type of game he or she runs.) So my take is that Forgotten Realms has supplanted Greyhawk. I can't see GH as anything other than a superfluous, vestigial setting (with Blackmoor and Mystara).