Well if bitcoin mining becomes ilegal we will see the hash splitted between a lot home miners, and well there is no way to stop them all, even hitting those wich does spend a lot energy at home, there are already clean energy that are used to mine as well. Mining has became a game which early and big fortunes can grow without almost any risk. I wont be alive to see the last bitcoin mined, but one thing is sure it will be a massive cost to let all the hash to protect the coin, now imagine the fees cost as well the bitcoin value it will be.