Nobody said anything about the CPU, I've gone from two cores to quad to now... I dunno, whatever the hell an i7 is. 16? My screens have also gone from 1680x1050 to 1920x1080 to 2560x1080. I just took umbrage with the idea that you need to spend hundreds on your GPU to get good performance. People see the 3-monitor setups and RGB lights and streaming equipment and think that's PC gaming, but it's not.
Games are more efficient than ever today. The worst era was definitely the really lazy Xbox 360 ports at the end of the 00s/early 10s. Those games would turn my GPU into a space heater.
Might have left a couple of steps out of your upgrade path then:
> I built my first computer 15 years ago for $500 using a $120 GPU. 10 years ago I upgraded it with a $120 GPU. 4 years ago I upgraded that with a $120 GPU.
If you've also upgraded your CPU to a relatively recent one, upgraded motherboard to suit, dropped some more RAM in it, maybe added an SSD, and your "4 year old $120 GPU" was actually a high end one that you got cheap, then sure. Your 15 year old PC case can play modern games because it's actually got a modern computer inside it.
Games are more efficient than ever today. The worst era was definitely the really lazy Xbox 360 ports at the end of the 00s/early 10s. Those games would turn my GPU into a space heater.