I think the Apple ARM chips are a good example. They're fantastically more efficient and fantastically powerful. We _could_ take this incredible platform and say "we can probably do personal computing on 3-5 watts if we really focus on efficiency." But we don't do that. With more powerful chips, websites and apps and operating systems will get less efficient, bigger, more bloated. If there's any slack in the system we'll just take it up with crap. The chips will be faster next year so why bother making things more efficient? Repeat this process forever, and we eat up all of our efficiency gains.