Consumer loads are mostly limited by single threaded performance, though software is increasingly written for more cores. Best choice for a consumer used to be dual core, now it's probably quad core with thermal limited boosting on less parallel workloads.
It's only prosumers doing lots of transcoding or rendering, or CPU intensive VMs, that typically benefit from increasing cores above 4.
Whereas the server space prizes throughput and much of its workload is trivially parallelizable. On the server, higher single core speed mostly just decreases latency for small tasks; if you're happy with the latency, you can get more cores working on shared memory and potentially get big wins in perf.
There are diminishing returns though, scaling up boxes is expensive and unless it's being forced by software licensing or architectural models, things like Hadoop and Spark for spreading the load across a whole cluster are increasingly attractive. This helps solve the I/O throughput problem too.
The big difference is in the E5/E7 chips.
E.g. with my last laptop change, I went from a 35W TDP i7-M to a 15W TDP i7-U, while still gaining some performance.
Laptops with 35W/45W CPUs still exist, but they use e.g. the quadcore i7-6700HQ.
I'd hardly call that old.
And that's not an apples to apples comparison. The 6700HQ is a quad core part, whereas the 7500U is dual core (both of them feature hyperthreading, so 4 and 8 threads respectively).
Given equivalent software, the 6700HQ will outperform the 7500U in multithreaded workloads.
If intel needs 20% more cores to match the MT perf on the high end then it really isn't an advantage.
This, of course, leads to a tremendous advantage in server CPUs -- if you have capable cores at a lower wattage, you can add more of them. Hence the 14 core E5-2690V4 @ 135W at 2.6Ghz vs 8 core E5-2690 @ 135W at 2.9GHz. So in just four generations from Sandy Bridge to Broadwell, no TDP change, roughly the same or a bit better single thread execution but almost double the core count. If you are willing to drop a tiny bit your base clock further and your TDP higher, perhaps 10% less single thread performance, you can get a hulk of a 24 core E7-8890V4 at 165W. And that's where the big profit is -- currently.
Now some unfounded nonsense: what if Intel is not pulling these crazy prices out of their sorry behind but there's in fact some reality behind them? It just bothers me that the price of the 24 core chip is so close to the 6*6 times of a 4 core chip. It could be a coincidence.