I5-2500k at 4.6ghz on big air that I bought when it came out was running great with a GTX 1080. Upgrading to a Ryzen 7 1800x@4ghz was a marginal perf game (and ~0 FPS gain in most games). The m2 SSD from a SATA SSD seems like it helped the most.
Still running a 2500K here too, paired with a GTX 1070. Absolutely no qualitative issues with any games I play. Interestingly though I do notice its age when programming - builds take noticeably longer when compared to my work laptop (XPS 13 w/ i7-7700). Could be due to the higher core count I guess (not sure what level of parallelism my builds exploit beyond the parallelism offered by Make).
Indeed, I'm still running an i5-2500K as my primary gaming machine (with a GTX 970) and upgrading is hard to justify. It doesn't even have USB3!
Slap a USB 3.0 card in the thing and keep running it. The state of the art hasn't advanced all that much since Sandy Bridge.