Astrud Gilberto, Neil Young, and Stevie Nicks, are all extraordinary talented people,
Gilberto is quite talented, both in delivery and as a composer, but her delivery is amazing, and she has songwriting skills, mostly, she had the advantage of being pretty, and a very capable singer, and someone who was novel.
Neil Young is a talented songwriter, and a master guitar player, his vocal skills are secondary to his skills as a songwriter, and guitarist.
Nicks does have an unconventional voice, but with a phenomenal power to her voice. That's just natural.
And Astaire? No one would call him a great singer, but he was one of the best and most talented dancers of his era, the same goes for Gene Kelly, while they were stylistically very different, they were fantastic dancers, their singing abilities were secondary to their abilities as dancers.
The key for all of these, if you have a radio face, and a great voice/talent, you're probably not finding commercial success (not in the modern era - thought there are exceptions), if you have an average voice/talent and a really pretty face/body, you might find commercial success. If you have an average voice/talent and an average face, you're more than likely, not going to find success.
All four examples you listed had something extraordinary about them.
Giberto, Beauty - and an average talet, Young, an extraordinary talent and an average face, Nicks, a unique voice, and an extraordinary face. Astaire, a multi-talent, and a handsome man too.