[Chibi-Chan]

^I wasn't arguing that there wasn't an impact on popular culture from Ayu in the 2000's, but that what she is known for by the general public is music she released during 1998 and 2000. From A Song for xx until Duty, which was cemented by her peak in 2001 with the release of A BEST which is again her music from the mentioned period. So at least for my friends who were born in the late 80's/early 90's Ayu was very present in the 90's. Same goes for Britney. I'm almost going every month to a 90's party. Most songs which get played are songs from the second half of the decade, simply because the audience is between 20 and 30 years old and they would not remember songs from 1991.
Ayu will never be popular among teenagers again, and in this case she really is too old. Not because she is old in general but too old so that a 13 year old girl could identify with her. And that's perfectly fine. And when watching a Namie concert at least I don't get the impression that her fans are a bunch of teenagers but mostly people being between 20 and 30 years old.
Most of Ayu's fans have the same age from my impression, which makes sense since most of her fans are long time fans by now and have grown with her. And to be popular again Ayu would need to appeal to much more people being between 20 and 30 years old. Because there is no way to appeal to teenagers and people who are between 30 and 40 years old have other idols, mostly from the 80's and early 90's.
So all I was trying to argue with was your point that Ayu isn't popular because she's from the 2000's but Namie is becaue she's from the 90's.
I simply don't agree because for an audience being between 20 and 30 years old, both were from their point of view as children pretty relevant in the 90's and think of them as big artist from the 90's.