Hell yes a zero carb diet is sustainable -- look at Martin Levac and some other people here who eat zero carb.

The better question to my mind is "are you a person who can eat zero carb forever?" My answer to that is no -- I prefer life with broccoli and don't feel very good on zero carb. Most people's answer to that question is no. Doesn't mean everyone's answer is no.

And for most people on this forum an even better question may be "Is zero carb better for weight loss than low carb?" That can get into a whole lot of areas -- sustainability, diet vs lifestyle, rate of loss, health impact, etc. That, in fact, seems to be the question that is argued.