I think some of you are expecting too much precision from this word. "Pop" connotes a tangled web of ideas. It sometimes connotes popularity, simplicity of structure, and accessibility, but it also connotes love, sex, adolescence, commerce, consumption, mass production, and a rejection of academia and dry formalism.
If the word is worth retaining, then that's because we recognize that these ideas are connected in deep and confusing ways. If it proves useful to think of "pop" as a genre, then that's because we recognize that certain cultural artifacts are implicated in this nest of problems, and that we can help shed light on the problems along with the artifacts by thinking of them together, under a common name.