I agree with you. I have piles of books detailing very old crafts like processing game, tanning hides, trapping, hunting. Knapping stone tools, weaving baskets, raising pole barns, distilling whiskey at home, making clothes, mending clothes, herbal/wild medicines. The foxfire books being a great example of how we preserve this knowledge.

Almost anything modern we've learned is going to be well-documented IMO. We may not be referencing those documents any longer, but they exist, and will exist for a long time.

And also agree that modern methods tend to be better, or at least the best version of the old methods is widely known, thanks to youtube etc. Primitive Technology on youtube, for instance, was able to launch himself into the iron age without bringing any modern tools along. But the access to modern informational sources makes him a primitive powerhouse.

You don't know what you don't know. Just because you know of some things being preserved doesn't mean all knowledge is being preserved.

For example, the US has lost its ability to do all sorts of manufacturing.

Businesses don't tend to document internal processes publicly. As those businesses die out and the people who worked for them retire, there is no one left to pass the skills along.

That's why the Army always keeps one tank factory open. So the knowledge and skills are not lost.