[criminal]

It's very rare for a musical group or singer to stay in the business for that long.

And like. That yearly average of 5m+ is calculated by dividing Ayumi's sales with 9, right? After another 9 years, the average could be about 100k, considering her declining popularity and the fact that she maybe wouldn't like to release an album every year.
I wouldn't hold my breath waiting for her to become the biggest selling musical act ever to hit the scene.