At least for now.
Here's my take on the low carb thing. There are 18.2 million diabetics in the US. A large percentage of them don't even know they are diabetic. The rate of Type II diabetes in younger people and even kids is rising. Millions more are at risk of diabetes due to genetics, etc.
One of the best things a diabetic can do (I'm one, so I speak from experience) is to throw away the Am Diabetic Assn exchange plan and start eating low carb. They can reduce or eliminate their neeed for meds completely. So-called borderline diabetics on low carb may never turn into full blown diabetics.
People will start seeing lower carb - not NO carb, but a lower carb diet including whole grains and beans, as a healthy way to eat. I think it makes sense for people with health issues to watch what types of fats they eat (maybe more chicken than beef, cool it w/the whole fat dairy, etc.), so a modified Atkins such as South Beach may be more appropriate.
I think we are looking at lower carb as a way of life for millions of people for a long time to come.
I think you are right about this not being a fad. In my house, we have eaten the low carb way for over seven years. People stick with this for more reason than just weight loss. It has incredible health benefits.
As opposed to before we started to eat this way, everyone in my household now has excellent cholesterol ratios, blood pressure, triglicerides, and blood glucose levels. We each had worse initial cholesterol tests for the first 6 months or so while our bodies were adjusting, but since then the blood tests have been consistently good time after time.
I also got my energy back, lost my migraine headaches and chronic indigestion. Even my seasonal allergies were minimized. This is a very healthy diet for young and old alike. The affect on sales that is being seen by certain companies is not happening because this is just another fad diet. For many of us, it's become a way of life.