Wherever one stands regarding the question, it's easy to see that preserving disappearing languages as spoken languages is futile. Young people are naturally not interested in speaking dying languages, it's the old folks who want to preserve them. Yes, they can teach the language in kindergartens and schools, but often they can't even find proper native speakers as teachers anymore when there simply aren't many left. It's a form of necromancy, languages evolve and die naturally, long term we can't do anything to prevent their deaths. We need to preserve history like you say. But even historic records can be and have been re-written. Imo we should be most concerned with the cultures and attitudes that shape our current languages.