It does, however, become exponentially more difficult to double-spend, which is the whole reason we have proof-of-work security in the first place.

Yes, however, it already is enormously difficult to double-spend. If this were the actual motives of miners adding ever-more hash power, we ought to discuss at some point how secure is "secure enough" and at which point the added security has a higher cost than benefit.

Otherwise the whole thing sounds like the paperclip maximizer justifying its actions: "But if I burn this forest, we will have even more paperclips!"

The only motive of miners is profit. This is by design. How else could you incentivize a bunch of random people to find partial SHA-256 collisions?

Security is ensured by the economic incentives baked into the protocol. Security is the by-product of economically selfish behavior.

Which is why cryptocurrencies won't convert into more energy-efficient schemes - after all, they see the exponential waste as feature - and we'll be stuck with this energetic disaster until some authority finally puts a stop to this charade.

"Some authority" will use violence to stop mining locally, and it will immediately move elsewhere.

This is a nearly pure market system, driven by economic incentives. Cryptocurrency mining is the best and highest use for the scarce resource of energy. Once it becomes unprofitable to mine, then the hash rate will stabilize and the energy will be used for better purposes. Marginal producers of hash rate will be priced out of the market.

I don't think many people think that. They think more hardware means more money in their pockets.