Oh that's true of course; and hence lies the problem. All we are getting are slight performance increases in the last 12 months. However, in the last 7-8 years, we generally got 70-100% performance improvement at the same price every 12-15 months, or a 2x price reduction for the same performance.
Several times this trend didn't hold true, however. This happened when 9800XT replaced 9700Pro, 7800GTX replaced 6800Ultra and again resurfaced starting with HD5800 and GTX4xx series, which both flopped in terms of delivering 2x the performance at the same price or delivering the same performance at 2x lower price. For instance, HD5770 was actually slower than the HD4870 and priced higher when it launched. And frankly, GTX470/480 are nowhere near 2x the performance increase over GTX280/5.
No matter what card ever comes out in the mid-range, it will always be judged against 6600GT in my eyes, a card that beat out a previous high end 9800XT and cost less. So far, neither AMD nor NV have been able to deliver a killer mid-range card just like the 6600GT was back then. Even now 12 months later, GTX460 is hardly a compelling reason to upgrade to for GTX275/4890 owners. Hopefully HD6000 series raises the mid-range performance 2x from HD5770 - it's about time.