There's nothing stopping multi-PDS replication right now. It would work fine. No one would know that content is available elsewhere, is kind of the main chicken-and-egg problem.

Hubble is starting to come online, which is a network cache. With excellent storage and runtime characteristics, cheap to run. I'm not sure if this is in the same https://microcosm.blue banner-head, but the same developer, as another incredibly fantastic network service that's open for use, that's open source, that other people can also run themselves. https://atproto.com/blog/introducing-hubble-a-public-mirror-...

I agree that yes the PDS's hopefully start doing more mirroring too. It'd be nice if they would themselves also attest that they have seen it too.