For many of us who were adults in a culture before computers were a common place aspect of daily life, computers can still be emotionally and intellectually problematic. We are the technophobes who had to be led to computer use with friendly metaphors and cartoon characters. However, the generation of young adults about to leave high school already have no memory of a time before sophisticated computer games and mobile phones. Metaphor may be as irrelevant to interfaces of the future as instructions on how to use a domestic telephone are to us now. If metaphors that don't work now can be overcome by hard work on the part of the user, and in the future metaphor may be irrelevant, then the question arrises as to whether or not metaphor is needed even now.
All metaphor requires the user to first understand the metaphor.
This produces a cognitive load on them. Then they have to deal with inconsistencies in the metaphor, adding to the cognitive load. Perhaps it is already possible to to implement an interface which reflects a mental model without use of metaphor. It would seem that this could also be a great advantage in other regards, freeing design styles from the existing default use of menus, buttons and icons. This could allow greater opportunity to engage the user intellectually and emotionally.