Great replies so far. I'd add that if people want namespacing to actually happen they need to collaborate on an RFC (https://github.com/rust-lang/rfcs/blob/master/0000-template....). Ideally someone would post a pre-RFC to internals.rust-lang.org so it can be improved before formal submission.
Any such RFC needs to take in to account previous discussions (and appreciate that the crates.io team is small). E.g.:
https://internals.rust-lang.org/t/pre-rfc-user-namespaces-on...