I wonder if we could elaborate all of the costs of this aspect of the other proposal. I know it makes the proposal cross cutting, which you've highlighted as a problem, but it seems like a fairly narrow change to cargo that would be manageable.
I note that the notion of linking and associating existing crates with a namespace are not necessary features of the other proposal: you could allow every crate to become a namespace using a newly introduced separator without allowing moving existing crates between namespaces.
The big disadvantage of this proposal is that existing projects (crossbeam, tokio, etc) would not be able to use this feature. This seems really detrimental to me. But I think a version in which every crate can be a namespace could be stripped down to an MVP that does not introduce very much complexity. Maybe I'm wrong.