It isn't often that one encounters an entirely original theory of language loss, but I chanced on one in this comment by one TB Tabby on this blog post:
This is how languages die out: Over time, every single word in the language becomes slang for something dirty. People didn't forget to speak Latin: they just got tired of all the snickering whenever they spoke.Posted by Bill Poser at July 4, 2007 07:59 PM