Can you list your assumptions? For starters, are you assuming constant power production and constant power price? Because neither of those will hold whatsoever on the timescales you're talking about.
It is not a perfect model by any means that is why I called it "rough 'back of the envelope' estimates" . All values static and based on "today's" numbers:
https://bitcointalk.org/index.php?topic=694401.0The rising worldwide power production will "help" us use a lower percentage of the overall power production.
The rising cost of power will "help" us use less power.