That is a very good question.
What would I have listened to if Glay or Gackt, or even the Pillows, flew past me growing up? What would I have done if JPOP queens like Ayumi, Namie, or even Nanase, not been my gateway to music overseas?
Japanese music is something that has become a core part of my very being, allowing me to learn about the outside world, as well as how the internet works, which in turn, led me to a career in Information Security. I can't imagine something else that could fill its place.
If I had to guess, I would venture to say that American Metal would have filled in the void temporarily, before drifting away from that last bit of teenage angst, and succumb to just listening to whatever comes on the radio.
I say that because when I first listened to Japanese music, I purposely went out and taught myself how the internet works, in order to browse forums, blogs, and site's where the P2P sharing wave of the 00's existed, in order to get my hands on as much as I could.
I'm pretty sure that I would not have done this for an American Metal CD that I could just go and buy down the street, and so eventually I'd lose interest.
I'm sure I'd just stream my music at this point, just like everyone else.