The truth is that there was no definition of rock music to start with. In the following decades there was less and less of an agreement on what constituted rock music, as its purveyors swung wildly from jazz to world-music. By the end of the century, rock music included artists who played mainly electronic and digital instruments.The problem is that “rock music” was never a definition of the music, but a definition of the audience. Rock music was music for young white rebels. As those young rebels grew up, it lost its “young-only” quality. As times changed and people accepted the Establishment (maybe because they had fewer reasons to attack it), the “rebellious” quality was reduced to a mere search for originality. Thus rock music evolved into music for white originals. The music itself changed dramatically, but the audience that rock music had created basically continued to exist, mutatis mutandis, across generations. Thus an identity could be find in the audience, not in the stylistic attributes of the music.
The media were largely responsible for determining what that audience listened to, and therefore what rock music was. The media’s defining power was already evident in the 1960s. Hendrix happened to be classified as a rock musician mainly because his records were reviewed in rock magazines and therefore sold to a rock audience. He might as well have been classified as a blues musician, or even a jazz musician: had his records been reviewed mainly by blues magazines, his audience would have been the blues audience, and therefore he would have been part of the history of blues music, not rock music.
Ultimately, the reason some musicians were considered “rock” is that rock critics and rock historians (such as me) wrote about them. The only consistent definition of rock music is, in a sense, that rock music is what i am writing about.
The only viable definition is a “use-based” definition: rock music is the set of all musicians that the rock community writes about. Thus Klaus Schulze (an electronic musician) makes rock music, but an electronic musician raised in the classical community does not make rock music, even if their styles are very similar: the difference between the two is that the rock press writes at length about Schulze.
It is not the listener who defines what is rock music, it is the reader.