When the Sino-Tibetan languages (Mandarin, Wu, Cantonese, etc...) invaded northern India, they displaced and eventually replaced indigenous languages. Because this happened in pre-history, we have no idea how many languages disappeared, but if history is any judge, it was likely dozens or higher. This is parallel to what Indo-European languages did to Europe, the Near East, and western India. Until the 21st century, everyone thought that the original languages of Nepal (and the Tibetan mountains) had vanished forever. Gone the way of the Fir Bolg and the Picts.
Until several years ago, three Kusunda speakers were discovered in Nepal. This excited local linguists because, according to what people thought, it was a presumed-extinct Sino-Tibetan language that we would get a second chance to study. What we got was so much more. After the first in-depth survey of the language was published in 2005, historical linguists noticed it was not Sino-Tibetan at all! It was a language isolate, unrelated to any other languages on earth. Subsequent analysis revealed its the last link to the world of Central Asia before the pre-modern invasions.
Kusunda studies have only begun but native Kusunda speakers are very old. When they die, so too will their language go with them. It's a vital connection to a world so long ago and can tell us how the aborigines of Nepal viewed the world, thought about religion and life, and how they mastered the jungles. It also tells us things like ancient trade routes, wars, and cultural stratification. In short, we're missing out on a golden opportunity here. The Kusunda are doing fine in forests of Nepal. The Kusunda do not need us.
We need the Kusunda.