Currently namespace aliases are defined as simple identifiers, while namespaces can (but are not required to) be multi-part.

Unclear why there is this distinction; it's not possible to tell from a single-part name whether the name references a namespace or an alias, so why prohibit multi-part names (i.e., names containing a dot (".")) in aliases?

There are scenarios where a namespace is long that an application may want to use a shorter name but still represent some level of hierarchy in the naming scheme.