The canonical datatype not only identifies conformance resources (which is indeed an overlap with uri or Identifier), but also makes explicit that elements marked as canonical have certain kinds of behaviour (they resolve to .url element of conformance resources, you can chain over them in a search, etc). We'd like this to remain explicit, and using Identifier would no longer do that. We appreciate the fact that this explanation cannot easily be understood from the spec, and that it is worth adding some wording of explanation around it. Also, the definition at, say, StructureDefinition.url (and other conformance structures) does not mention "canonical" but talks about "logical", it would be good te rephrase this as well to clarify.