In maven central, in my anecdotal experience, namespaces usually either distinguish forks of the same library, or to group related libraries. There has never been, in my experience, a case where I wanted two packages with the same name in different namespaces.
I think a better system of handling unmaintained crates; with the possibility of reclaiming a name would go a long way with the author's use case. I don't have any great ideas here: package with no update in some time (a year?) is eligible. Someone requests to take over name - the author is notified and has a period of time to click an "I'm still here" button. If they don't - new author gets the name. Some 'super version' number is incremented so crate authors opt into the new or old foo. That last part is where I'm the most hesitant.