Earlier quoted context omitted.
> You don't have to match the energy burn, just the hash rate (so efficiency matters). Of course, but I don't know how anyone could be a lot more efficient that current miners, who are in a rat race to increase the efficiency of their mining operations. They probably think about energy consumption every waking hour of the day. > You also don't have to do it all at once, you just have to be faster than the network (If…
Re your PS, there will still be an old and new consensus blockchain no matter how fast you calculate the replacement. (that's why it is a "chain", the head depends on all previous values)
Yes, of course. In fact, at any point in time there are always multiple blockchains, one for every miner burning energy to add a new block.
However, only the longest blockchain (with the greatest proof-of-work) is valid. As soon as there's a longer valid blockchain, the miners discard their individual work-in-progress blockchains and switch to the one with the most proof-of-work.
That longest blockchain is the Bitcoin blockchain. That is the blockchain one would have to modify to alter the transaction history in the Bitcoin network without forking.
FWIW, I don't think we disagree; I feel like we've been talking about slightly different things until just now.