Who says that Ayu is loosing popularity...?

[Melon Panda]

She's Ayumi Hamasaki - even if she's not at her peak era, she's still considered "queen/empress." While Koda Kumi, Ai, and YUI are popular now, their popularities still don't come close to Ayumi's popular at her peak. Ai and YUI seem more "trendy" than "popular" to me; people might remember them as artists of their era, but neither really stands out (though you can argue YUI's potential). BoA is unique in her own way and will probably be remembered in Japan and Korea as a kind of standard (like Ayumi), but I don't find her nearly as popular either. Koda Kumi will probably be remembered for her sexuality more than anything else.

To sum it up: stars like Ai, Yui, Koda, etc. are famous, but Ayumi is a legend.