You are probably right Bobby, duets, especially cross genre duets sell as they create an interest from different parts of the music consumer.
The original statement might have had some legs if the situation, and the statement, were made before pop music and pop music cultures exploded in the 50's and 60's. Since then not only have pop stars careers been reported and disected by various media, including mainstream, but there are also his recordings to give testament to his career. Prior to this it may only have been certain elite's who recorded history, not any more, even this is out there somewhere now.
I dont think time is going to change the way pop stars careers are viewed, there just isnt the oportunity to re-write something that so many people commentate on. (as a side note, people are regularily trying to re-write Northern soul's history and their part in it, and undo any positive contributions they may have made by making themselves look daft, this is with a tiny fraction of the commentators that Rod Stewart would attract.)
I believe his back catalogue alone will show future generations of music investigators that he was just a guy who had a distinctive voice, came across as an alright kind of bloke, but essentially a karaoke singer who pulled off a fantastic career by using other peoples songwriting talent.
Nice subject