But how many other groups did that as well? To me, Sabbath is the first metal band. Many earlier groups contributed to its creation, but Sabbath embodied it. There were plenty of other driving bass and drum parts, and also lots of distortion before Zep. But The Who aren't considered metal. Nor is Deep Purple, Hendrix, or Cream. But they all have influenced metal quite a bit.
Metal owes a great deal to Zep, but they also owe a great deal to classical composers. No one says Bach is metal.
I'm not trying to bash Zep by any means, as they have earned their spot in rock history. But it seems we're trying to put them where they don't belong, and I don't think that's how they identify either.
Anyway, my two cents.