What's your beef with the GTX 580? Maybe I took your post the wrong way. The purpose of the 580 is to BE FERMI. If anything, there is less relevance to the 570 since it is only in existance due to being incapable as a 580. This holds true to any castrated part.
As far as there being no purpose to releasing a faster part... What are you talking about? Then why do we need a next gen? Apparently not to be faster.
If nVidia were to release a GTX 585 6 months ago I imagine you would see a ton in peoples sig's and rig's around here
That it's a crappy card for the price, perhaps? Inefficient, loves to suck power, very expensive, etc. Your "castrated" part argument makes little sense, if any. It has no bearing on a product's price/performance, and the GTX 570 is easily the better card of the two because of its performance for the price, heat output, and power consumption.
The purpose of the GTX 580 from the beginning was not only to finally be able to bring a full GF100 core to the market but since NVIDIA knew AMD would release a card faster than the HD 5870 they needed to retain their single card crown. It's a high-margin product for NVIDIA to use as a marketing tool so fools can buy it; nothing more.
We need a next gen because it brings with it significantly higher performance at previous price points as well as lower noise, lower heat output, and lower power consumption. We also need it for so-called enthusiasts to give NVIDIA money so we can have our slightly slower but otherwise much better cards.
Sure, as soon as you get higher than the Radeon HD 6870 you lose price/performance, but diminishing points of return get HUGE when you look at the GTX 580.