I don't really see why a word couldn't become extinct. But, if you don't know about the word, you'll never know that it's actually extinct. And, if you remember it, then it isn't extinct. It's sort of confusing, I guess, but I think words have probably just disappeared before.In this day and age, though, of transfer and storing information, I think it could be quite difficult just to "lose" a word and let it become extinct. You'd have to actively try to get rid of a word, I would guess. That would be hard to do, though. We have the internet - we have dictionaries - we have the merging of the two in online dictionaries.