How is a mirror not solving the problem of data loss if a single instance goes away?
The problem isn't a single instance going away. The problem is what happens when for whatever reason the owner stops maintaining the project. This is a common problem and despite all the bluster and buzzwords, the IT community hasn't really found a solution. Torrents are the only kind-of-solution, but they are not ideal for something that needs to be constantly updated.
Zeronet may help for content that need to be updated