I honestly think you don't really know what you're talking about. Do you honestly actually think that a next gen card is going to be 15% faster than the last? That's utter nonsense.
As for the average speed increase between generations, 25-50%? I call a huge amount of BS on that.
Since I had a 9800, nVidia and ATi have both brought cards out that were roughly 2x the performance of the last generation.
They wouldn't bother with a 15% increase. It's laughable that you're even willing to suggest that.
Seriously, I don't know anyone that would be interested in next gen cards if they go by your numbers. Maybe you're really bad at maths or something? As for the 4870 512MB, that was faster than the original GTX260, and is neck and with the latest GTX260. The 4850 was around the same speed as the original GTX260 yet it was priced at G92 price points.
ATi is a business, it knows how to run its business. It doesn't make sense to price as high as possible. No where near the same amount of cards would get sold if they did.
Even if you want to factor greed in to the equation, then they'll still reduce the price and sell bucketloads. What OEMs are going to choose nVidia cards of ATi's when they cost far less while offering similar performance?
Are you drunk or something?