My interpretation is that it's not a title, but rather who she is by marriage.
Agree that when Charles becomes King then the Prince/Princess of Wales will supercede the Duke/Duchess of Cambridge titles. For George though, I wonder if he might at some point inherit the title of Duke of Cambridge - I think that's normally when the current title holder dies (Edward will become the Duke of Edinburgh when Philip dies), but given his position and that by the time William becomes king George will be an adult, perhaps it will be passed to him upon adulthood, or marriage.
As a point of interest, I think most know that Camilla is currently the Princess of Wales but has chosen to use the title Duchess of Cornwall instead.
I'm also very interested in what will happen to the Dukedom of York. The title is normally reserved for the second son/second in line to the throne, which he was for a time, but is no longer.