In JSON, the order of fields in an object is unspecified. In order to preserve the order of Statements, Qualifiers, References, etc, we need to represent it explicitly in the JSON output. One way is to give each element an index field. Another way is to have a special member in the associative map that is just a list of keys, giving their order.
This blocks "WikibaseDataModelSerialization with what we do with WikibaseLib" and although we have totally removed the Lib Serialization I would not consider this task complete in regards to WikibaseDataModelSerialization..
Right now we are doing the "Another way is to have a special member in the associative map that is just a list of keys, giving their order." one so I guess we should probably just keep doing that!