> I think governments would be far more likely to ban new rigs, but may let grandfathered rigs continue to operate.
This seems irrational given the ever increasing efficiency of hashes per energy.
It's only irrational if you think the government cares about maximizing hashing efficiency. It's perfectly rational if you recognize that in most (western) legal systems there is a difference in the ease and acceptability of stopping someone from taking a new action and the allowing them to continue taking that same action. This is due to various legal underpinnings, but also makes perfect sense in that allowing someone to invest millions and then pulling the rug out from under them is very different from telling them they were never allowed to invest those millions in the first place.
Building codes are an excellent example of this. It's perfectly legal to maintain, sell, lease, etc. a building that would be illegal to build today.
You have to factor in the amortized cost of specialized hardware over it's lifetime, and a few other factors. But it's a good approximation.
BTC tried to solve this with diminishing rewards for mining. The problem is BTC's market cap doubled far faster than the rewards halved.
But I think you are wrong at the high end. If BTC was worth 100x tomorrow, the current miners cannot just push a button and draw 100x the power. I think governments would be far more likely to ban new rigs, but may let grandfathered rigs continue to operate. If that's the case, I can see miners becoming quite profitable.