Though it is compatible with earlier versions, I don't see any reason to keep sorting at all, especially as a default.  The reasonable default is that whatever order the user specifies is what the tool does.

The backwards compatible argument is specious.  The ordering was never guaranteed.  The only reason that we had sorting was to make the XML generation deterministic, and that is something we would still have.

I vote against the permanent API expansion with an unhelpful default.  The sorting is no longer necessary or desirable.  Appropriately, we never guaranteed it, leaving us the implementation flexibility to make exactly this change.

AFAICT, no XML user has ever expressed an interest in having attributes appear in alphabetical order -- for that matter, I can't recall any HTML user having expressed this preference (instead, they prefer to put class and id tags ahead of other tags for example).