It's not a mirror, it's a proxy. If someone encroaches on their free speach and shuts down their hosting account that "mirror" will not save them.
Just saying, this is an important distinction to me and I've been hosting tor nodes since the 2000s.
Archiving information, and making it available, is sometimes more powerful than anonymous proxying.
Especially if there's an anonymous proxy available to that archive. ;)
As long as they have the private key they can move it to new hosting infrastructure without issue, and the same onion address will still be operational.
"Mirroring" is a term also used when a single source publishes data in different mediums (technically in this case we're talking only about the internet but the internet is full of different protocols so I'll call them mediums). For example there are websites that mirror their content to Geminispace or in this case make it available as an onion service.
You are correct that this solution does not prevent problems if the server goes down. This particular approach aims to reach a larger audience, while your idea of mirroring enables resiliency.
Both approaches have their use cases and can even be combined too!