Blah, blah, blah 1080p. Bump that to a 4k monitor and the i5-2500k will have almost no issues.
Fps limit is set by the cpu, not the gpu. Fps onscreen is the gpu. Cpu doesn't have anything to do with resolution or detail levels, that's all gpu. So a 9900k and 2080ti will get 60fps at 4k, the I5 2500k will be right behind it. The first will be gpu limited, the second cpu limited, but on a 60Hz 4k monitor, it doesn't matter which is limited but one for sure will be.
Even the more graphically intense games at 1440p are going to stress even high grade gpus.
Yes, there are plenty who use Sandy-Bridge and Ivy-Bridge still, it's a matter of not everyone plays the latest and greatest must have popularity contest games.