Quite a few of the IDOLS who didn't win, including Clay Aiken, Tamyra Gray, Matt Metzger, and Frenchie Davis to name four that come instantly to mind, have parlayed their exposure on the show to nice careers on stage and TV.
But NO ONE can predict the public's acceptance or rejection of a new personality. MGM learned that with Lucille Bremer: great introduction in MEET ME IN ST. LOUIS, a starring role opposite Fred Astaire in YOLANDA AND THE THIEF, and within two years, her screen career was over.
The last two IDOLs have not had supersonic record sales, so you can't predict whether winning that title will launch them into the heights of stardom or merely show them to be flashes in the pan. All the show can do is provide an introduction. The rest is up to fate really.