Well, you could also solve it by saying that the post slash names are unique. ie. There can't exist zardeh/serde if erickt/serde already exists. Then the author-name works as a logical checksum, and you aren't any worse off than you were with a global namespace.
The purpose of a namespace is to make it possible to disambiguate two otherwise identical identifiers. If you force package names to be unique across all namespaces, then you don't have namespaces at all, you just have a single global namespace where you're forced to prepend an author name to the package name.