This got my attention today:
Ultimately, I think the question is really a distraction. One of the great strengths of XML is that the instances exist independently not only from individual schema definitions, but also independently from the schema language of the day. [From: Don Box]
True indeed. In case of industry shifting to RelaxNG (very unlikely), instance documents would survive nicely. As long as there is a schema, that describes an instance document, everything is fine. When the connection is lost somehow, we can't talk about instances any more.
XML without a defined schema is no better than CSV. It's not even easier to parse.