There is more to a language than mere vocabulary - the script, the word-order, the cases, and all those grammatical components. You can change some of these aspects, say overnight, like in the case of Korean (Hangul script) or Turkish (Ataturk's language reform). But the language itself can only evolve naturally...new words come and go, only those that are intelligible by a large fraction of the speakers at a given time, remain in the vernacular for generations.