This is something that keeps me worried at night. Unlike other historical artefacts like pottery, vellum writing, or stone tablets, information on the Internet can just blink into nonexistence when the server hosting it goes offline. This makes it difficult for future anthropologists who want to study our history and document the different Internet epochs. For my part, I always try to send any news article I see to an archival site (like archive.ph) to help collectively preserve our present so it can still be seen by others in the future.
We’re actually well beyond RAID arrays. Google CEPH. It’s actually both super complicated and kind of simple to grow to really large storage amounts with LOTS of redundancy. It’s trickier for global scale redundancy, I think you’d need multiple clusters using something else to sync them.
I also always come back to some of the stuff freenet used to do in older versions where everyone who was a client also contributed disk space that was opaque to them, but kept a copy of what you went and looked at, and what you relayed via it for others. The more people looking at content, the more copies you ended up with in the system, and it would only lose data if no one was interested in it for some period of time.