This is a diametrically opposite problem from what the parent comment was talking about. They were saying that P2P is bad at sharing new popular content from one user to many, which is patently false. You are worried about old content that hasn't been accessed in years and decades disappearing. This is a real question to think about.

Right now reddit and twitter bear the burden of maintaining access to entirety of old content. Reddit even has a system to "archive" posts older than 6 months to make storing them on server easier. In a decentralized network, no one user has that responsibility. What can be done about it? Maybe we need to reconsider our idea of "permanence", tone down our expectations that all content will be accessible forever, even if no one accesses it. Maybe a censorship-free P2P network would need some sort of sunset system anyway, because otherwise it will fill up with useless spam (the same way Usenet was made useless because it became 1% posts and 99% binaries). Maybe data hoarder enthusiasts will run archive nodes with much larger storage dedicated to preserving old post history. Maybe you can add a filecoin-like system to your P2P network, where you pay $0.01 to guarantee that your comment remains online for 10 years, $0.02 for 20 years, etc. Not recommending it, just saying there are options.

Do note that neither reddit nor lemmy are immune to such bitrot. If reddit goes bankrupt and shuts down servers tomorrow, all that content will be gone as well. Maybe archive.org will manage to save a snapshot, maybe pushshift.io will have a backcopy, but what about all the posts made since pushshift API access was revoked? They'd be gone. As lemmy instances go in and out of existence over the years, what happens if this instance and the ones that got a federated copy of this post all go offline? This post will be gone from history as well. Its continued existence can only be guaranteed if users on the new instances years in the future go back and view it here again before it disappears.