It's sad, but how many languages have we lost over the centuries? How many variations of Italian or French have we lost due to migration? As a Deaf person, this is readily apparent in local flavors of ASL. I noticed this with deaf people I came into contact with at RIT as a student there. There were many slight variations. You could tell who were new to the school, or those who's been there for a while. The ones new…
I don't know about languages, but for dialects, I think the answer is the same as the number of people who have died over the centuries.
No one speaks a language exactly like anyone else, so ultimately, every person has a slightly different dialect. There's no way to preserve them all. Your analogy about preserving living trees in a book is apt.