It's worth noting that, if you need something which runs on really low power, ARM have their R and M series processors. So even if the A series did become really power-hungry, the other two lines presumably wouldn't.
Eg, the processor in the BBC MicroBit pulls about 10mA @ 2.5V or so. Of course, it only runs at 32MHz or so & has a whopping 16kbit of RAM, but you can run one off a pair of AA batteries for a week or two without ever sleeping.