From what I understand, Led Zep were by no means the opposite end of the spectrum from manufactured pop. They were assembled by Jimmy Page from industry professionals, plagiarised a lot, and operated largely as a shameless cash generation machine. And the results were fantastic, of course.
If there was a formula for their success, nobody has been able to repeat it.
I know they plagiarized some old blues stuff, but all musicians do to some extent, and those old blues tunes were forgotten then and would still be forgotten if Zep hadn't brilliantly rearranged them.
