Andy, your model takes into account only the ''creation'' of new words. But in any living language, words fall out of use, too.
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Imagine a country where a constant number of ''conservative words'' is created each year, but where these words have a half-time of 100 years, that is, e.g, only half of the words used in 1600 were still in use in 1700.
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Such a country would have the same distribution of conservative words as [[Conservapedia's Law]] implies - but the overall number of conservative words becomes constant after a while...