Not sure where bitcoin got dragged into the conversation. Also not sure how BTC becomes less efficient by design. The hardware BTC is mined on has improved exponentially over time moving from general purpose cpus to gpus to custom asics.
one of the major key principles behind bitcoin is that the hashing becomes more difficult over time.
hardware can't constantly keep parity with difficulty.
Now, there has been a strong positive correlation between time and total network hashrate, so for all practical purposes, difficulty has (and likely will continue to) increase over time.
That said, if half of all miners went offline overnight, block times would approximately double, and the next difficulty rebalabce would go dramatically lower in an effort to maintain 20 minute block times.
It's not really about efficiency as it's about computing power and available miners.
If it's not economically beneficial to mine (costs more than reward) miners will stop mining leading to the difficulty decreasing which in turns makes it profitable to mine again.
e.g. you can't bullshit your way into finding this number, 000000000000000000024394a1f3cb1a0c16e601a2bd5910635bb2468d2ba316, without expending a huge amount of energy, while you can verify very quickly how many guesses it statistically took to find it.
An attacker can't just use words to spin a manipulative narrative, or cut of the head of an organization with a targeted attack. They actually have to commit massive numbers of joules and bit flips. And if an attacker actually acquires that much control over mining power, suddenly they realize they're too heavily invested in the network to want to harm it.
In the age of increasing generative AI, proof that you have some tie to real world cost is an increasingly valuable trait.