It won't be possible to save languages going extinct. Even if two or three people start writing a Wikipedia in such a language, it will die out as a spoken language, eventually, not later than it would without a Wikipedia. I think it's nice to have a corpus of encyclopedic articles in such languages, but more important for the goal of Wikimedia to make knowledge accessible to all people of the world, is that there is a useful Wikipedia in at least one language any given person can read. I would estimate that we won't ever reach 300 (open) Wikipedia language versions, because many of the smaller ones will be closed sooner or later due to permanent inactivity, and that's perfectly fine.
Th.