That's a good point. I think there's a tendency to conflate the idea of "lost knowledge" with "inactive practice" when it comes to dying languages. Record, preserve, archive information about a language, as much as possible, but to say that all languages must remain "alive" means that we're saying that some portion of humanity must expend the time and effort to remain multilingual in languages that they may not receive practical benefits in. Maintenance costs of "in-brain preservation" are relatively high, and in doing so that's allocating time that may or may not be spent in pursuits that are beneficial to the actual people whom some want to be vessels for a dying language.
Languages don't "die", they are smothered by a society hostile to the community that speaks them. That may be outright genocide (as in the article), or it may be something as simple as forcing children to go to attend school in a language foreign to them, that offers zero recognition of their mother tongue. The kids may have to learn the content in a foreign language, and they are denied even so much as a hour-a-week class on the literature of their native language.
A system that is intent on eradicating the cultural heritage of a people is not trying simply to do only what is beneficial for the kids. =/
