All the great metal riffs have their origins in Black Sabbath's music. Tony Iommi was/is a riff machine.
Then you had Priest & Maiden.
Then you had Metallica & Megadeth.
Then you had Pantera (I love Pantera's opening/chorus riff in A New Level).
The progression goes on until today.
I heard it asked if all the riffs have been used and now it's just recycled and/or adapted riffs from other songs. I don't know. I'm sure mathematically there may be an end to the creation of riffs at some point simply because of the number of notes in music and the number of combinations there can be with notes, scales, harmonies, etc.