I think the solution for the new PC is going to be a combination of both. 4 fast as possible cores; maybe 8. Then massive additional cores at lower clock speeds and simpler design. Most likely implies a hybrid NUMA design with additional performance specs and turntables for the host OS and user level software(games). Probably a few modes of operating. Automatic management and succeeding levels of the OS taking over management.
Think of the slower processors as something akin to a floating point coprocessor. Programmers are lazy. Until the hardware exists and they know they will see a benefit they don't want to program for it. Programming for massive unknown hyperthreading is a hard topic. They will always choose the lowest common denominator.
But having both fast/few and slow/many/efficient changes the game. The fast/few is at the end of it's performance curve life but will live on. It's nice to have a few cores around that run two to three times faster for difficult programs. But having an undetermined number of extra processors sitting around.... programmers won't be able to control themselves when they want more performance.
I can also imagine this extending to mixed architectures such as x86+Arm like we have heard rumors about. CISC+RISC in the same computer; blasphemy. FPGA compute boards and graphics card becoming coprocessors and integrating on a more equal level with the CPU's.
P.S. The future of operating systems is a three tiered approach: Hypervisor->OS->Programs. The hypervisor being something akin to a microkernel. What if instead of building compatibility into successive operating systems you could run them all together and forget about compatibility altogether. With additional software and drivers I can even see compositing. This means we can run FreeBSD, Redhat Linux, Ubuntu, Gentoo, Arch, DOS, Windows 3.1, Windows XP, Windows 7, Windows 8, Windows 10, and GNU Hurd, and multiple x86 and Arm versions of Android all on the same machine. Imagine all those with compositing extensions.... And once you get rid of the browser with Flash and Shockwave 99% of viruses disappear so administration actually gets easier and if you do get a virus you can always rollback; or rollout clones to test software. Also one can setup a file system with features like file versioning and encryption so that programs can't just hijack your data.
The reason I brought up the above is because one can further still extend the hardware with a clustering OS. A business could install such a specialized clustering OS on each of it's desktop machines in the building and then have it's own cloud. Imagine using something like that for rendering and processing video.