In other words, when a language no longer serves the interests of a powerful group it is likely to decline. Maybe in some cases, but I bet not in every one. I should think this would also occur when the powerful group was very small in number, compared to the general population. People would just go on talking among themselves, their language would change, and eventually the written form wouldn't correspond very much to actual conversational words. Or, as the article says, the powerful group could simply be overrun by outside invaders.