Old mining rigs are long ROIed, they still generate coins when they work as a heater, and since most of the electricity that is consumed by mining rigs become heat, if 1/10 of those electricity can become bitcoin, it is still more efficient than a pure heater. And that's why the network will never go down in hash rate, because old mining rigs can still work as a heater

Of course I still think the waste of electricity is huge in POW, I do see benefits of POS, but that model is quite close to company share holder, need to have further discussion about its implication