"Pop" makes sense to me as Maiden wasnt quite heavy enough. They rocked hard all right, but nothing like their genre-mates Manowar, who were heavier, faster, louder and more over-the-top than any of "those" types of bands (to the point that they parodied themselves). But metal split so wonderfully into so many small subgenres that one could take exactly what they personally considered "heavy" and find a band who dedicated their sound just to THAT. Rockin-out is one thing but achieving "heavy" is quite another. And this can be done in SO many ways. In all, the 3 guitars and bass formula worked for Maiden...as it worked for Molly Hatchett. But thats just good ol hard rock...Pop..if your brain needs to be pummeled, like mine, under sludge, crunch, chaos, doom, monster and robot vocals, torturous drones, satanic mumblings, and frenetic, dissonant noodlings and needlings.
A whole lot of teenagers who learned their chops playing Maiden grew up to make REAL metal. And for that I regard them very highly and will never get rid of those records.