The authors of model-dependent realism claim that what type of theory may not exist?
Model-dependent realism:  The term "model-dependent realism" was coined by Stephen Hawking and Leonard Mlodinow in their 2010 book, "The Grand Design". The Grand Design (book):  The authors of the book point out that a Unified Field Theory (a theory, based on an early model of the universe, proposed by Albert Einstein and other physicists) may not exist.
Unified Field Theory