ERD50
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
-- and in music you're absolutely correct, one generally builds on the body of work of others, but Elvis was not Chuck Berry in terms of being a pioneering influence on music the way we speak of people like Robert Johnson, Bill Monroe, or Armstrong.
Coincidentally, I have converted most of my CDs to FLAC files on a hard drive over the past month (I think I'll tackle the vinyl over the next winter), and I have several CDs of each of those artists you mentioned (the 2 CD Box set of RJ is everything he did, maybe they found one song since then). I've often thought that it was Bill Monroe who was really the most original - although Bluegrass builds on other styles, it really has some unique and (AFAIK) previously unknown components. I'm not enough of a musicologist to say that with any authority, just an impression of mine. I would say that Monroe's music was a bigger step away from the predecessors/contemporaries that was Johnson or Armstrong. But all are fantastic artists with a lot of originality.
It's all a matter of musical taste; like a poll in 2005 that had this somewhat obscure singer ahead of Elvis.
Well, Eva Cassidy was an amazing interpreter of some classic songs. And her sad story helped her to connect (posthumously) to the public. To get back closer to the OP, I'll take anything Cassidy recorded over anything MJ recorded, any day. As you say, a matter of taste and personal preference. But then, I can't dance
-ERD50