While I don't disagree with the central germ of your statement, I would question whether society at large still holds this view, or whether it is simply a minority that happens to be very vocal.
It is, of course, difficult to say with any certainty. And I may be a bit of an optimist.
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They didn't write the music, but they did write the lyrics. I'm not totally positive on the exact line there, but my understanding is that since their writing includes aspects of the musical portions of the show, what they wrote was technically the libretto. Though in that case it would include the guy who wrote the music. Who wasn't included but was instead credited separately with the music.
Perhaps the proper way to handle the crediting is to simply say that "they wrote the book and the lyrics and this other guy wrote the music." There's also the chance I'm thinking way too hard about this.