I mentioned the use case when it popped up in my head, but it does seem like a bug more than a feature with the way you put it, especially since I said in my opening post the editable install should resemble a regular one if possible 
It may be possible to move to require a rebuild to add new modules in the new standard. Most people (I believe) use editable with the expectation to convert the package to non-editable at some point. I think people will understand if we communicate this clearly in the spec (PEP?) process that it was never the intention to allow it, and the new behaviour may even help prevent bugs in the conversion. Some might still get annoyed, but cue mandatory xkcd