Yeah, ensuring availability over time requires dedicated infra. That’s basically what it comes down to. Torrents for the most part lack dedicated providers ensuring file availability. Web seeds exist, but the uploader or the tracker needs to have the resources to back their torrents with bandwidth and storage. Other decentralized solutions, like say IPFS, don’t solve the resources problem, because it’s not technical, so although you can pay to have content “pinned” in place on IPFS, or you can pin it yourself, that “pinning” requires a server, running off electricity, using someone else’s uplink to serve the content, all of which costs. If you don’t have your own server, and don’t pay someone else to pin it for you, it could easily fall off IPFS.
Syncthing could honestly help, I’ve thought about this a fair amount, although you’d still have the resources issues. Availability of content over syncthing or something like it would likely still be tied to popularity (how long are uploaders going to keep their syncthing folders full of specific content? how long will downloaders? In order for it to really work people would have to get in the habit of building out NAS’s and putting their libraries on syncthing forever, basically). It still has some of the same basic issues with torrent, but the dynamicness is cool for sure.