Didn't Intel recently juice up their Skylake i7-6700K desktop cpus to 4GHz, though. More cores and more cache has certainly been on the frontiers of pushing the cpu. I'm wondering if they can't resume getting the clock speeds up, too.
Higher clock speeds are absolutely still possible, just not in CPUs with high numbers of cores. More cores = more heat, and dealing with heat is one of the biggest challenges when designing a CPU.
For servers, doing more things slightly slower is (usually) better than doing fewer things faster, so Intel usually puts core count over clock speed for their Xeon CPUs. For a desktop system that's unlikely to be doing more than 3 or 4 things at once, you can prioritize single core performance and higher clockspeeds.
It's also considerably more efficient to have more slower cores than fewer faster cores. Reducing power consumption and reducing heat is a win-win in a datacenter.
Desktop applications tend not to be very optimized for multithreadding, games will still usually use 1-2 cores max which is what those CPUs end up being used mostly for anyhow beyond that professional applications also tend to optimize around 2-4 threads/cores.
For the few desktop apps that do scale well 6 and 8 core i7's are available but at those prices you better get a mid range Xeon with a desktop socket since it's unlikely you'll be overclocking your cpu in that case anyhow.
Intel will release the Core i7-6950X later this year. It has ten cores and will apparently cost around $1500. Not sure which use case except one-uppance and bragging they're targeting, but it will probably fly with the right workload. Video encoding and 3D rendering are the only things I can think of that will scale easy to that kind of CPU, although you'd think people with those needs would buy Xeons.
Well the "X" (Well technically E as in Broadwell-E, Haswell-E etc.) is part of their Extreme line it's meant for "enthusiasts" that really like dumping money on high end CPU's and overclocking the hell out of them this isn't a workhorse, at that price you can build a full dual socket Xeon for about the same price as the CPU (if indeed it will be 1500$, I'm betting on more like a 1000$ like their previous top of the line Extreme series).
Video Encoding isn't that well multi-threaded either as it's some what linearly dependent (you can't just encode random frames without having the previous frames/key frames done for references which means you can usually do 2-4 frames at the time so run off becomes and issue when you have more cores than frames to encode), 3D rendering is also a mixed bag depending on what type of raster and post processing you use you will get substantially different scaling between number of threads vs pure clock speeds.
In any case video encoding and 3D rendering (at least the ones that one will do on a desktop) will benefit much more from multiple high end GPU's than from an increase in CPU cores, the more or less conversion is that GPU numbers give almost 1 to 1 scaling where additional CPU's (cores) give you 0.3-0.5 on average in the best case scenarios (there are a few cases where CPU scales almost as well as GPU but they aren't that common).
