I am under the impression that the technical advances in the graphics apartment in the last years have not been particularly ground-breaking.
So they have to leave the high-end not optimized in order to keep people buying new hardware. In my opinion, if a 580gtx can't run it, its not because its too demanding, its because the game is just not optimized. Just notice how after a development of 3, 4, 5 years... games suddenly run a lot better with patched by a month or two after release... they have to release a hardware-demanding so hyped people buy new computers, even when they don't need them (raise your hand if you bought a pc for Battlefield 3 thinking it would be the next big thing...

)
They are trying to convince us that they invent a new wonderfulous technology every year that surprisingly improves the old generation performance by exactly 10-15%...
Most games are multiplatform, so i would not expect a big graphics jump until the next generation (and still, based on what they have showed so far, it does not look very impressive anyways).
This is just my personal opinion, of course. And that said, Shogun 2 had a quite good performane in old graphic cards if i recall correctly.