What I am saying is that it's way too soon to say that they'll go down in music history with the Mozarts and Beethovens of the world. The people who grew up with Zep necessarily carry a lot of emotional weight along with the music, which pretty much prevents any neutral perspective on the merits of their music. I think Dave Barry said it best:
But that doesn't matter. What matters is that when I'm singing along to "Shut Down", I'm no longer a middle-aged guy driving to the laundry to pick up my shirts; I'm seventeen, and it's a summer night with tantalizing possibilities of adventure and romance hanging semipalpably in the humid air, and I'm cruising the roads around Armonk, New York, and even though the vehicle I'm cruising in is my mom's Plymouth Valiant station wagon, which boasts the performance characteristics and sex appeal of a forklift, I am feeling good, and I am stomping on the gas pedal (not that this has any measurable effect on my mom's Valiant) and imagining that I'm at the wheel of a Stingray, singing triumphantly along with the Beach Boys as we roar past the Dodge 413...So I don't care how many times I hear "Shut Down", or "Little Deuce Coupe", or "Fun, Fun, Fun". They're always welcome on my radio; I'll go back to that summer night any time.
But there won't be anybody like that left in 50 years - at that point, it'll have to stand or fall on its own merits. All we can do is sort of wait and see how it's received by generations to come...