I think people repeating a single melodic line can still create good songs if other aspects compensate. Every pop artist even The Beatles have done this before. I canít like The Rolling Stones without accepting that limitation
However I do feel that when you are speaking of 5 star artist, basic minimum is that you have a significant amount of songs where there is a flowing melodic line. What I mean by that is that each vocal line is different but connected to each other. Ideally a melody line should work as an individual sentence within a paragraph.
Reason why this is better is that gives melody a sense of character and journey. IMO this unites The Beatles with Mozart and Tchaikovsky (although Mozart is a superior melodist the fact that Beatles have something in the same ball park is an achievement in of itself).
Other five star artist such as The Beach Boys, Gabriel era Genesis and The Kinks also have significant amount of songs that does that as well. It also part of the reason why I recently downgraded the Rolling Stones to a 4 star band, because they do this far less often.
In any case my issue with George Gershwin is that I came in with significant high expectations. Prof Robert Greenberg called him the American Franz Schubert (where significant amount of classical music expert considered him the greatest songwriter in western music)
I really expected to hear music transcending ordinary pop music based on that description. In the end I heard music that sounded like ordinary pop music
Maybe I will learn to like Gershwin songs with repeat listen but I doubt I will be blown away by it.