One implication of this PR is that the user would be unable to create the description of a non-rdf resource without also creating the non-rdf resource itself.
We need to be able to support the case where the non-rdf resource lives in a federation but the resource description lives in the repository.

We were never able to create a non-rdf resource description on its own.

curl -v -XPOST -H"slug: image" http://localhost:8080/rest/collection?mixin=fedora:datastream

You're not making a non-rdf resource description there. You're making a empty binary resource. You'll find you get the same result if you give it any non-RDF content-type.

We were always creating an empty binary resource -- and I'm fine with that. It's not a huge burden to create the empty binary. Providing the redirect is also very easy to do, and provides seamless retrieval of the external content in a way that's transparent to the user.