In metal I disagree, there is literally nothing as important as riffs. Especially if we limit things to 1970 through 1993. In modal jazz or ars nova, I will happily admit that riffs aren't important, but metal is based on riffs. Some sub-genres emphasize the importance of riffs more than others; in power, black, and sometimes death metal it's sometimes acceptable to play a lot of straight, unsyncopated rhythms, in trad and thrash it rarely is, but riffs are still the defining aspect.
But that's literally a matter of rhythm is it not? Aka a fundamental domain of the riff. Two notes in two different places = two different rhythms = two different phrasings = two different riffs.