Overall, your point is valid but there are certain games where the difference in CPU performance between an i5 and an i7 or a 6-core i7 is huge. In other words, you'd be losing a lot more than 10 fps in performance with a slower CPU (i.e., your GPU would be underutilized to its full potential).
Eventually any 2015 GPU will become too slow but a 2015 i7-6700K @ 4.8Ghz will still be potent in 4-5 years unless something dramatic happens in the way games are designed on PS5/XB2. Chances are a 2015 CPU will survive 1-3 GPU upgrades. After having an i5-2500K, my next CPU will either be a 4C+HT or 6-8 Core. There is no way I am going to bet that over the next 5 years there will be 0 next gen games that take advantage of the extra threads. We'll have to see what happens with DX12 though as it could reduce CPU demands by allowing the API to take full advantage of the 4 cores in the i5 thus utilizing it much more. I am not sure if DX12 will help the i5 or the i7 more to be honest.
It depends on the games you play and your resolution. In some cases the i5 6600K is barely hanging in there with a stock i7 2600K.
That's a pretty pathetic showing for the i5, considering i7-2600K is a January 2011 CPU and i5-6600K is an August 2015 CPU. People who buy an i5-6600K and then spend $70-100 on an after-market CPU cooler make me facepalm.
In some games, like
Crysis 3, even a max overclocked i5 cannot match a stock i7.
The point is next time you go to upgrade your GTX770 GPU, as long as you have a solid i7 like
2600K/3770K overclocked to 4.5-4.8Ghz, you an keep using that CPU for another 2-3 years. i7 6700K OC isn't much faster in games.
i5-2500K? Not aging as well at all in games that benefit from 4C+HT.
Of course it all depends on what that extra $100 buys in terms of GPU performance. If we are talking i7+R9 290 vs. i5+GTX970, I would pick the i7 system all day. If we are talking i7 + Fury vs. i5+GTX980Ti, I would pick the GTX980Ti system. But long term, I would spend $100 on an i7 now because I am more confident that modern Skylake i7 or 5820K or similar will last 4-5 years which means $100 extra is really just $20-25 per year extra.
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Anyway, not sure why this GPU discussion on Black Ops turned into a CPU discussion considering we have Black Ops 3 CPU performance being discussed in that section of the forum. I think CPU related discussion should take place there since there is already a separate thread for it.