Aside from the pressure from AMD, I think there's another force at play here. There are quite a lot of people now that just use a web browser on their desktop or laptop. For them, a 5 year old (or older?) CPU is good enough that they don't know they are missing anything. Especially if they have enough memory and an SSD.
You can see a counter-example in mobile phones. Their processors are continuing to make considerable strides in power-constrained performance, and 2-3 year old phones regularly wind up getting perceptually slower as the software advances in performance requirements.