You don’t need to go crypto to get there though, IPFS and similar exist and work. IPFS itself is kinda slow, but Iroh is aiming to be a more efficient alternative that solves similar problems. There are also protocols based on BitTorrent.
The way these work is basically:
users connect to relay nodes
relay nodes connect users directly
users continue communicating directly w/o any servers
Then you build stuff on top to keep everything in sync. No servers, aside from the initial connection, which means minimal risk of anything ever going down. If relays go down, anyone can set up another and people reconnect.
The problem is that step 3 is quite complicated, and there are a ton of technical complexities to synchronizing information at scale w/o a central authority. Mastodon/Lemmy/ActivityPub gets around this by having each node (instance) be a complete copy of everything that node cares about. You get a ton of duplication, and eventually that means costs pile up. With a proper decentralized system, there doesn’t need to be nearly as much duplication since you can always hop through some peers to find what you need.