The thing I don't like about the way bitcoin mining works is that it's an arms race that leaves attacker and defender on equal footing - quite unlike a lot of crypto, where adding a bit increases the defender's work linearly but the attacker's exponentially. By its nature, then, we have to keep burning off more in value than someone could make with a 51% attack, regardless of improvements in tech efficiency.

Of course, lacking a better proposal, it could be that this is the best we can do. I don't have to like everything about every piece of technology in the world...