Gurg :
5820k + MB+16 DDR4 2133: $727
4790k+MB+16 DDR3 1866: $599
4690k+MB + 16DDR3 1866: $519
If you buy 4790K or 4690K you will be stuck with an under performing CPU and outdated memory. Given its track record Intel will be changing the CPU pin configurations or RAM slots on MBs to facilitate DDR4 on next generation of CPU/MB. The only technological upgrade route for those that buy a 4790k or 4690k now is the GPU.
Yes and no. Technically, the next gen is Broadwell. We've only seen a tablet version of this CPU so far. There's no telling if Intel is even going to release anything beyond that with Broadwell, and the only thing it has to offer is slightly better iGPU performance and lower power consumption.
The next gen after that is Sky Lake, which will support DDR4 and PCIe4, so it will definitely feature a new socket and new chipset. It also won't be released for at least another year. Since most things on the PC are more limited by storage than anything, I don't see these making any significant difference anyway.
However, I would hardly call the 4790k an underperforming CPU. The 4690k is already top of the gaming CPU chart. The 4790k offers no extra performance in gaming, just like the Haswell-E series. Sure, Haswell-E has more cores, but no game right now utilizes more than 4 cores. We are entirely GPU limited in the game space right now. This may change in time, but by the time it really makes a difference, it will be time to upgrade again anyway.
The increases Intel has offered recently have been increasingly irrelevant. Intel itself is becoming decreasingly relevant. They're still top of the heap for PCs, but they're not quite as powerful as they once were. I built my older sister and her family a Core i5 2500 machine several years ago, and it is still more computer than they can use. Even that old iGPU is more than they can use. This is the case with most home and business use. There's just no software out there that uses what we have.
Heck, the reasons to upgrade to the newest software are even decreasing. The biggest reason is security, but even in that case, it is affecting fewer and fewer people. If the software isn't getting upgraded, what is the use of upgrading the hardware? My sister still uses Office 2007, and has no plans on moving away from it.
Face it, the high end desktop is becoming more and more elitist. Fewer and fewer people actually need that high end.
Now, if you're talking the server side, that's a whole different story.