Though it belongs solely to them and has inestimable value to their people, they do not hoard it. In fact they are often eager to share it. What can we learn from these languages before they go extinct? And why should we lift a finger to help rescue them?I am less certain.
As the last speakers converse, they spin individual strands in a vast web of knowledge, a noosphere of possibilities. They tell how their ancestors calculated accurately the passing of seasons without clocks or calendars. How humans adapted to hostile environments, from the Arctic to Amazonia.
As a person fascinated by languages and historical linguistics, you might expect me to be very strongly on the preservation side of this argument. And certainly, the loss of any human knowledge is a negative. But I don't find the arguments compelling that we should get all that worked up.
It is charming, and worthy of adventure plots in children's fiction to think of wise old grandmothers passing on tribal knowledge to willing descendants, including language and life lessons which figure prominently in the heroine's coming-of-age. But to learn a language fully one must grow up with it as a conversational language. When no young children are learning a language, it will die. Governments encouraging Gaelic or Welsh in school can slow the tide, or perhaps even bridge it over to a time when a native language revives, but this is quite rare. More importantly, the speakers themselves often do not regard the knowledge as that important. They grow up with two languages spoken, and increasingly discard the less common one as they get older. The Auld Tongue gets dragged out for special occasions and ethnic festivals, but lapses into disuse. I learned to say grace in Swedish as a child, and I passed that on to my two oldest sons. It's all quite charming, but I don't see that language fragments have provided any especial wisdom. In fact, the word gagn meaning "gain" is now archaic in Swedish, making the little I know even less useful. Do the fragments of Chemehuevi that still survive in Arizona make us all wealthier in knowledge? Does it even make the possessors of those fragments wealthier in knowledge?
Most of the knowledge of endangered languages is self-referential, circular, and bound not only to a particular culture but a time period two generations past and more. We can extract some knowledge of language relationships as an aid to guesses about history, perhaps, but our genetic knowledge already begins to exceed that and will soon overwhelm it. Then also, the tribes with endangered languages are often isolated and poor. Do we propose to keep their children isolated and poor so that we may study them like animals in a zoo? No thanks, pal. I'll learn one of the 10-20 most common languages and get a job, thanks.