^ that was my unscientific belief as well.
I think that 5E will present a real choice for WotC -- either go back to something more like 3E in critical ways (multiclassing, etc.) or revise and refine 4E. I do believe that 4E split the fan base in a way that was pretty unprecedented. Although that's not the only system that did it -- I think the nWOD did as well, although as far as I can tell it mostly succeeded in just killing the fanbase off.
Despite strong sales, I don't know if PF really is the new standard-bearer for 3E. As has been illustrated on these boards at length, there is still a great amount of both skepticism and annoyance with their basic design philosophy. Further, I think reviews complaining about very shoddy mechanics in their Ultimate Books of X lines (those are just the reviews I read on Paizo's own website) have undermined their ability to supplant 3E for the "hardcore" 3E fans. I think unless PF/Paizo alters its design philosophy to a significant degree, there will still be a significant part of the 3E community (say 20% maybe?) that is active and relies on old 3E books or homebrew or smaller press productions.
For what it's worth, I have always felt that 4E was really the spiritual successor to AD&D. Sure, there are lots of different mechanics, the resource management is totally different, and so forth. But, in AD&D saying "I'm playing an Elven Ranger" told me a ton about your character. In 3E, it tells me very very little, given multiclassing, ACFs, and just various build things.