Yes, as Nostr and Freenet disconnect the storage location from the document identifier. Thus your link keeps working, even if the servers change.
The problem with the Web isn't a lack of cheap storage or people willing to mirror content, it's that none of that matters when the links keep breaking anyway and nobody can find the content anymore.
For how to do it right, see Usenet, the messages are held together by unique message-ids and the protocol was designed for distribution. Thus archives could be put together from random backup tapes and even 45 years later, you can browse and read the discussion threads.
Your average web forum just disappears when something goes wrong and even if somebody has mirrored the HTML, they'd have a rather bad time trying to recreate the original database, since the HTML is really more like screenshots of a GUI than the actual content.
> The web is just the library but on the web
Many countries have a legal requirement that every book published has to be send to the national library. We also have ISBNs that uniquely identify books. We have nothing like that in place for websites.
> they don't disappear quite so easily.
A local copy doesn't preserve it on the Web. Even if you wanted to preserve it on the Web, you can't, since the original domain belongs to somebody else.