I'm not sure I buy the analysis, for many applications the single threaded performance of a 8700k vs an X5670 is doubled, e.g. ~6000 vs ~2600 in geekbench single threaded. You also can't draw conclusions of single threaded performance from multi-core comparisons due to turbo boost.

It's also worth noting that a X5670 cost ~$1400, so you're comparing ~$2800 worth of CPU to ~$350 worth of CPU in something like an 8700k, and that doesn't cover platform cost differences. Another way to look at this is that 8 years after the release of the X5670, you can get better performance for ~10% of the price, that's not bad. If you did plop down ~$2800 today for a CPU or two, you'll have 28+ cores at your disposal.