Ritchie Blackmore was quite happy to credit many of his riffs to previous artists and ripoffs, including the most famous one, Smoke on the Water being Beethoven's Fifth played backwards. No one seemed to care back in the day (drugs woz good).
As I recall Led Zeppelin gave credit to the origins of their inspirations and copying, so I'm rather interested why Michael is suing now as executor for Randy Wolfe's estate. Randy was a member of the band Spirit that toured with Led Zepp in 1969.
Apparently Wolfe was peaved enough in 1997 to publically show his hurt at not being credit for his 1968 Taurus song being ripped of by Led Zepp album release in 1970.
This current suit was filed in 2014 after Jimmy Page prepared to release redigitised versions of the albums.
Word is that the song has earned way more than US$600m.... amazing.
I reckon someone should approach Steppenwolf and tell them that AC/DC have ripped their song: