I'll toss a comment in from a layman: Isn't it also true that cores are adding more and more specific instruction sets to boost common workloads? Graphics, encryption, etc.

Then you have improved architectures, memory/cache access, inter-core communications... there are more ways to improve a chip.

I agree with your snetiment, for general workloads. I have a 4 year old Haswell and it's working like I imagine the i5s today would perform on a basket of workloads.