IF governments were to adopt such a strategy it would not be the end of bitcoin, it could easily be moved to a different method such as POS or even the difficulty could just be scaled down so that it can be mined with lower electricity usage.
You can't reduce electricity usage by "scaling" down difficulty.
People will add more and more miners as long as there is profit to be made, the only thing that truly dictates how much power all the miners are using is the block reward (+ tx fees) and the price per coin.
This would be very hard to do, it's not easy to pick out who is mining and who is just using a lot of power in their homes. So they'd have to develop some sort of system that could diffrentiate people that are just using a lot of power and people who are just using a lot of power. This is something that power companies couldn't do to find marijuana growers so I wouldn't think they could do it in crypto.
It'd be horrible if they could do it, but I doubt it could be done effectively.
It's pretty simple actually, you check the total kwh consumed a month and you pick up homeowners with more than 2000kwh per month. That's only for two s9 but it is also 6 times the average consumption in Italy.
Then you look at the hourly usage and if that guy is burning the same amount hourly continuously it's definitely a miner.
Marijuana growers have different patterns in consumption from day to night and with improvements (led bulbs) they've started to use less and less energy for a plant cycle. Miners face different problems, once the cost per Thash decreases the difficulty goes up and you need more miners to make the same money and you end up with the same power bill.