GPUs don't "go obsolete" in 3 years, even in mining usage - the HD 7870/7950/7970 from *over 5* years ago are still effective profitable miners, and the R9 290 from about 4 years back or so is STILL one of the top Ethereum miners on hashrate (only surpassed reliably by the GTX 1070, GTX 1080ti, Titans of the Pascal generation, Titan V, matched by the R9 390/390x and SOMETIMES topped by Fury and Vega cards).
if your electric cost for a GPU rig is 1/3'd of your gross income from that rig, you have high priced electric by mining standards.
So here you are assuming or speculating that the new gpu generation will continue like before, seeing low gains x old gpus, if nvidia launches a gpu which mines eth at 80mhs and cost $400, nobody with sane mind will buy any old gpu.