Running at higher clock speed, the Due has to suffer higher power consumption compared to its 8-bit counterpart.
That's somewhat questionable. Smaller transistors, lower voltage, etc. And that's before you get to the "it runs faster, so you can spend more time in low-power shutdown modes", or "The clock is all PLL controlled, so if it's using too much power because it's running too fast, just run slower!"
Board-wise, none of the Arduino boards are designed for extra-low power consumption anyway.
The way I see it, 8-bit CPUs are likely to continue to be sensible as long as you don't bump into the limitations of 8bit architectures. The AVR is designed around 16bit addresses, so as long as your program is less than 64k words, and your data is less than 64k bytes, it'll probably continue to be viable (architecturally, anyway. Whether it will be overly expensive because it was designed with older design rule and uses a bigger chunk of silicon, is a separate question.) When you get to something like the 2560 or the big xmegas, and have to start worrying about near and far pointers and such, it's time to start wondering whether it would all work out better on an architecture with 32bit addresses...