Let's talk about the American diet, for a moment. I don't mean, what Americans eat. I mean, what do most Americans mean when they "go on a diet?"
Almost always, they're saying that they're embarking on a new path for a short period of time, with the intention of going back to how things were as soon as possible.
In this case, sustainability isn't a question, it is part of the definition: I am going to do something that I have no intention of sustaining.