This is not a coincidence. SGML now has multiple inheritance. An element can conform to any number of base element types, each of which is in a different base architecture. Their primary purpose will be to allow the SGML parser to function. Enabling architectures will define enterprise computing.
Human civilization is an enterprise, too.
HyTime is only the first SGML enabling architecture for civilization as a whole. Many more SGML enabling architectures will be created and used. Most will simply formalize, in SGML terms, the way things are currently done. Some will be created more thoughtfully than that. All will be subject to evolution.