I think that the Internet Archive is doing just fine in terms of being used as a way to save important parts or contents on the Internet
That's not a decentralized system but just a free digital library. We are talking about the distant future here, what makes you think that the forum wouldn't be online but that library would be? The solution shouldn't involve a website that requires its domain or hosting to be paid and updated every year or so because in that case, it could go offline sometime in the future just like how this forum can as OP said.
So, the storage should be decentralized and it should be working without needing any maintenance or something.