And we've had tiny, tiny increases in performance with each, and increased prices to match (the largest jump is with the 79xx, and the price severely inflated on them). Video cards were like CPUs, the 7970 would have replaced the 6970 at the same price.
Look at the performance of what you can get for your money from both companies. At a given price range, the amount you get hasn't increased much at all. This is especially true on the lower end of things.
In the processor space, we know when something new on the top end comes out, it's going to be $1,000 and no one is going to buy it. in the $200-$400 range, we're going to get what is most purchased (by enthusiasts), and every release is going to be a substantial improvement. Up until AMD released the 79xx cards, pretty much the only card that beat the top end of the previous generation is the top end of the new generation. Now they have 2 skus that beat the top end, but the price was adjusted upward to match, keeping similar price for performance.
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