I don't see what the big deal is, lots of the musical composers of their days were considered "pop stars" and their music was played on the radio. Musical songs were "popular music" and if composers known outside the musical theater world can help to bring that "attitude" back a bit, than I think that's a good thing. And there are good song-writers out in the world that aren't based in Musical Theater.
But I don't think we should think of mainstream singers, but mainstream song-writers/composers (there aren't that many really great singer-songwriters). Could Dianne Warren right a musical for example?
Of course they need to be able and willing to cooperate, work with people who know what they are doing in terms of structure ect., perhaps collaborate with a lyricist, and of course a book writer.
Also because someone mentioned Adele earlier, I think she talked about toying with the idea somewhere but wasn't really convinced yet or something like that.
And yes, Dolly is working of a musical about her life, but it's in early stages of development.