I'm sure AMD an Nvidia know something about upcoming PC games that has them pushing the amount of RAM in their upcoming cards. It would be my guess that it has something to do with the lowest common denominator going for 512mb of ram to 8gb of ram in November. I can just imagine how much ram usage we will be seeing when current games that are designed for 512mb consoles use up to 3gb of ram on the PC already.
First, you're arguing the future. We don't know what's happening in the future. But while we're on that discussion, VRAM is determined by:
1) resolution
2) anti aliasing
3) game assets
Again, I can't see any PS4 game having assets in excess of what the best PC exclusive games currently such as crysis 3. Crysis 3 uses 16k textures at the absolute highest quality setting (console versions obviously WAY WORSE texture quality...), and certainly those assets do not exceed 2GB in single screen resolutions. Much less 5GB. Additionally, MSAA is the second factor. With consoles reliant on other technologies such as shader based AA (FXAA, MLAA, SMAA) I don't see that being a factor unless you make it a factor (eg use override SGSSAA). Resolution for next gen consoles, unless i'm crazy, is likely to be 1080p.
And 8GB just isn't going to be needed for 1080p unless a game has something absolutely stupid like 2048k textures. And that day inst' even remotely approaching anytime soon, you'd be kidding yourself if you think any next-gen console is using anything remotely close to that.
Also, another obvious point is that neither upcoming console has 8GB of dedicated VRAM. I just want the end-user to have a choice. 3 or 6GB sounds perfect to me. Being forced into 5GB for a 780, not so much. Wouldn't you rather have a choice? GDDR5 is *not* cheap. Having that choice - being able to say, "Oh hey I use surround, I want 6GB" and having that choice would be great. Similarly, having the choice for 3GB would be great. Know what I mean? The alternative is not having a choice and an 800$ 5GB card landing. And let's not kid ourselves,
if it's 5GB it IS going to be in that 700-800$ price range as the baseline. With a choice it could potentially be cheaper for those using single screens - Again, GDDR5 is not cheap.