Given that they're shilling for Cadillac nowadays, I think you can make the case that advertisers are using music that they know strikes a chord with their target markets, rather than because it's necessarily timeless music - the folks who grew up with Zep are, let's face it, in their prime Cadillac-buying years.
We'll see how it goes in fifteen or twenty years, when Zep is shilling for Rascal-brand electric wheelchairs, or Depends, or whatever. Better yet, we'll see how it goes in 40 years, when all the people who fondly remember Zeppelin concerts are dead and gone - will their great-grandchildren still be listening to and enjoying Zeppelin? By way of analogy, how many of you boomers are big fans of Bix Biederbecke and Benny Goodman?
Great question. Bix's and Benny's albums are still on the CD racks and still sell. Past music leaves behind coteries who live and die for past masters. So there will be traces of Rock and Roll in the music market for a long time.
But it's doubtful that most of the music will have mass appeal after the the original audience dies off. Some of the music will have permanent appeal, though, in the same way that "Oklahoma" still finds an audience, long after most of the plays and shows of its day have been forgotten.
Future generations look at the music differently though. Most of it will be consigned to the rubbish heap, some will survive as middlebrow standards, and critics will heavily promote some albums that were scarcely known at the time. They will convince the next century that boomers were all listening to Captain Beefheart, or whoever their aesthetic idol is, rather than Captain and Tennille. And periods will tend to bleed into each other. Future generations may think that Elvis and Britney Spears were contemporaries.
This speculation on the far future is a lot like the depressing planetarium shows we saw as kids that got us all worrying about the end of the solar system, though.