> a system of mirrors would solve the problem.

A system of mirrors prevents a single node going down from taking the whole system (in theory at least, we've all seen plenty of times where failover goes poorly), but it doesn't do anything to ensure the long term survival of the system as people lose interest, lose the ability to participate, and sometimes die.

If you were around certain internet forums in the late '00s you might have run in to an image hosting platform called WaffleImages which was created in response to yet another popular free image hosting service locking down their embedding and ruining thousands of old posts. The goal was to distribute image hosting among community-operated mirrors, and it worked great for a few years. Over time though people lost interest while the rate of new mirrors getting added dropped to basically zero and eventually it fell apart.

Mirrors don't solve the problem, they move it.

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