Just mirroring doesn't change the retention policy (unless the term has changed its meaning in recent fifteen years), so it won't do. But this is moves the discussion to spoken language semantics, which I don't feel like pushing too far.

My point with the packages is that you need your own copy that you have control over, so they don't disappear unexpectedly. Pulling already-built packages from some other repository would be fine from this standpoint, though I prefer rebuilding them myself and keeping along with source packages.