Well, you did say it was achievable. How?

Hmm, I had a hard time finding the right adjective to complete that sentence. I see now that "achievable" wasn't quite right.

Of course, it is theoretically achievable. We know what needs to be done - it's all in Heinberg's Powerdown.

How is powering down going to reduce the population? Aren't the countries with ballooning population already powered down?
I think the misapplication of the power down concept has the potential to force population reduction (in an unpleasant way).  We certainly can and must reduce our energy use, and adopt new ideas about how we should live and how much energy we need to use doing it.  Such as electric light rail, higher mpg cars, increasing the insulation & efficiency of homes, etc.

But in the transition period, which must/will last a long time, we will need to develop new energy sources as well, and the most important one is probably nuclear.  We'll need to invest heavily in the electric grid as well.  The lead times are long, and if we focus all our attention on reduced use we may end up missing the opportunity - with disastrous consequences.  

The issue is one of rate - if we cannot power down (in a controlled fashion) as fast as the present sources deplete, then uncontrolled things will happen, including population reduction on a too rapid time frame.  Therefore we cannot just focus on powering down; we will have to look at sources as well.