Culture is a consensus. It just so happens that the region of culture nearest to me values the practice of sifting through obscure, niche, or distant sub-cultures for stuff that may be temporarily adopted in isolation, yielding social status points for the discoverer and early adopters.
The destruction of cultural artifacts previously unknown by the local mainstream is thus deplored, not because it had any real intrinsic value, but because it is no longer available for someone to show off and say, "look at this cool old thing that I found, that none of you culturally ignorant heathens have ever seen before."
As N-thousand-years-old artifacts are a scarce resource, whenever some hairy barbarian burns a book that no one cares about, he deprives some hipster of a critical opportunity to gain prestige in front of their friends for about 20 minutes.
Now, some people may become upset that elements of what they perceive as their own culture are essentially being used as toys and amusements for someone else's culture, but it is an essential element of culture that it cannot be controlled in such a way that such uses could be prevented. Culture is broadcast. You put something out there, and it is either adopted and rebroadcast, or it fades into obscurity. Once it is part of the culture, it isn't fully yours any more.
There is no protection for culture. If the consensus changes the meaning of something you liked to something you don't, you just have to move on to liking something else. If someone manages to make rompers for grown men fashionable, you just suck it up and wait patiently for the fad to die. You don't have to wear one, unless you want to participate in that part of the culture. And you don't have to rescue one from a thrift shop as a cultural artifact after it goes away. If you did, it would just be some thing to stick in a museum, so future visitors could see it and giggle, say "hmmmm", or vow "never again".
Not all cultural artifacts are worthy of "saving". For all we know, those unique manuscripts that might be put to the torch when the invaders march in are someone else's cultural equivalent of a Hitler Pepe meme or a man-romper silkscreened with Kim Jong-Il's face at 500% actual size.