It is no secret that if you do not ride a bike for long period of time, you lose your ability to ride a bike well. Can you still peddle and steer at the same time? Probably. But you won’t be as efficient or as fast at it.
The same applies to language. A spoken language vanishes every 2 weeks. That is an astonishingly fast rate. There are many reasons a language can disappear. The next generation learns a more main stream language in school and the older dialects fade away. A rural village that has kept their native tongue can be wiped out by natural disasters. How much is natural selection and how much have we willingly destroyed?