Merzbow is not pop music, or arguably even music at all.
>I threw all my past music career in the garbage. There was no longer any need for concepts like 'career' and 'skill'. I stopped playing music and went in search of an alternative. — Masami Akita
Pulse Demon is the only Merzbow album with any cultural importance, and that's mostly because the eye-catching cover art let it replace Lou Reed's Metal Machine Music as the default "check out how extreme this is" meme album. Only a tiny minority of music listeners will enjoy listening to it, and there is no establishment pushing it as the future of music.
Merzbow gets booked for popular-music festivals regularly, and plenty of other artists have credited him. The Japanese noise scene even inspired a Detroit noise scene through the 1990s and early millennium.
I don't care for the genre myself, but having been to events and seen pleasurable crowd reactions and met people who traveled some distance for it, I can't deny it has at least some real following. Who cares if it isn't discussed as the future of music? There are punk, metal, and hardcore subgenres that draw the same narrow audiences and don't aim to shake the world, but claiming that disqualifies them as part of the popular-music world would be silly.