Yes the availability will remain an issue but at least I imagine that solving other issues could make it less serious.

More specifically, the issue (a feature too but still) with torrents is how spread they are. It's difficult to know what is available and in what condition. There are dozens if not hundreds private trackers etc. This all makes it more likely for new torrents for the same content to be created multiple times, and overall seeding resources to be spread out across multiple versions of the same things. Some centralized public index might have helped everyone find things faster and prolong those things' availability as the result. What such an index might need to stay damage-proof and useful is unrelated to this discussion, but I imagine it might work as some blockchain and thus may not require much in terms of resources.

I didn't mean syncthing itself but some theoretical derivative that would have relevant features.

It would help to involve a kind of software infrastructure where users would choose how much resources (mostly disk space) they are willing to give in order to contribute to the overall availability of stuff.