Not all existing projects can transition to using namespaces here. For example, the unicode crate is reserved, so unicode-rs cannot use it as a namespace despite owning most of the unicode-foo crates. In other cases, the "namespace root" foo may be owned by a different set of people than the foo-bar crates, and folks may need to negotiate (async-std has this problem, it manages async-foo crates but the root async crate is taken by someone else). Nobody is forced to switch to namespaces, of course, so the damage here is limited, but it would be nice for everyone to be able to transition.
The RFC as written prevents certain namespaces from existing