The arch wiki could expand to support distro switching, with each distro having its own domain, and all that could link back to a general linux wiki. It'd be beautiful.

They already had it fine, now its out of their scope.

I think one reason the arch wiki thrives is that it IS a generic linux wiki.

The arch wiki describes the upstream package.

I say this because Arch tries to use the upstream software as-is, and describes requirements and helps make configuration choices.

Other distributions tend to make the choices for you, configuring and modifying upstream to fit the distribution.

With respect to a wiki, this would be another layer of documentation to add. Describe the package, describe the distro, and explain the configuration choices.