This is a point I see a lot of people miss about Bitcoin's power consumption. It doesn't have to use as much electricity as it does, as the difficulty is adjusted to mining capacity.
The reason why so much electricity is being used on BTC is a result of an arms race between miners, not a necessity of the technical specification. Bitcoin would still operate correctly with less mining capacity. What is sacrificed is that the threshold for mounting a 51% attack becomes lower with less miners. But again, the network itself does not need a nation-state's worth of electricity to operate.
A price crash that forces a bunch of miners off the network means that, not only is a 51% attack cheaper, but the spare capacity to launch one exists, and is just sitting there unused, losing money.
For security, the cost of the hashing needs to be greater than the value of being able to subvert the bitcoin network with 51% attacks.
If the cost of the hashing power is less than the value, then the incentive is to just buy the hashing power and do the 51% attacks.
So to the extent that bitcoin has any value at all, it's going to either burn an amount of electricity greater than the value of a network compromise, or it's not going to be secure.
How quickly would the difficulty adjust? My understanding is that there is an algorithm that slowly adjusts difficulty. If a big chunk of the ASIC miners went offline and mining reverted to individual GPUs, it might be a very long time before the difficulty reverted to something reasonable.
Bitcoin difficulty adjusts every 2016 blocks, nominally 2 weeks. Each time it can adjust by a factor of at most 4 (in either direction).
Many other cryptocurrencies adjust difficulty every single block.
No, that's not true. The problem isn't there protocol, it's the security argument. Bitcoin works with three people with one computer each, as long as none of those people has access to two more computers. As soon as one of the three can get three if their own computers, the chain's security arguments are broken. PoW doesn't work on compute, it works on scarcity of compute.
You don't think 3 people could figure out electricity and computers and networks whether local or even satellite links? That would be one hell of an apocalypse.
The reason why so much electricity is being used on BTC is a result of an arms race between miners, not a necessity of the technical specification. Bitcoin would still operate correctly with less mining capacity. What is sacrificed is that the threshold for mounting a 51% attack becomes lower with less miners. But again, the network itself does not need a nation-state's worth of electricity to operate.