Led Zeppelin "borrowed" from all of the blues standards that were popular amongst blues-based British bands in the mid to late 60's. If they weren't doing an outright cover tune like "You Shook Me" or "I Can't Quit You Baby" then they were "sampling" riffs and lyrics from those standards like many of the bands of the day did, including the Rolling Stones, Yardbirds, Animals, etc.
And as mentioned previously, Jimmy Page did ask Randy California to use the opening of Taurus for Stairway to Heaven. (I'm a huge Spirit fan as well so I've known this for a long time).
Led Zeppelin is one of the greatest bands of all time - there is no changing that no matter how many song bits they borrowed.
Besides, Green Day stole the riff from their song "Warning" from a song by the Kinks called "Picture Book" (1968).
