I do think you're underestimating the extent to which some of these people write or co-write their own songs, but I've been trying not to push that line of reasoning TOO far, since it becomes speculation. But I will say that in your above examples, Hilary worked extensively with Kara DioGuardi in the studio on her last album, Aly and AJ co-write almost all of their songs, and have exclusively written several of them (I imagine they do more of the initial writing than the producers they work with, from my brief glimpses into their artistic process from interviews etc.), and Paris confused me by getting more out of the same producers than they can muster with anyone else. Kelly is now being attacked because she's (said to be) writing her own songs, while we're currently having a debate on another koganbot thread as to what it means to say that Ashlee's involved in the writing process, combing through her reality show for evidence/lack thereof. (Unfortunately, very little of the recording process is actually documented on the show, and as Nia says, the editing is super sketchy/flat-out incoherent sometimes.)

Anyway, guess my point is that I don't think that any of the American pop acts you mentioned are interchageable at all!