There may not be much of a tendency for seldom-used languages to disappear A natural question (for those who think about languages at all) is: what number of fluent speakers is necessary for the language to be a rational continuing project? Assume that there is some mental energy exerted in learning a language, and continuing to use it over time enough to be "in practice," that is, to be one of those fluent speakers. Some portion of your language-using brain is freed up when you (a multi-lingual person) abandon one of your languages to the forces of erosion or rust or whatever. So: the more fellow speakers of that language you have, the more worthwhile to you are the costs of maintaining that ability. On such assumptions, it is reasonable to expect that there is some number of speakers X which is the minimum for a sustainable language. What is that number? David Clingingsmith thinks it is rather small: steady state (a language that is neither growing nor shrinking...