Aww, I think fans should worry less about her sales overall.
She's a mature artist, and like every artist (notable exceptions being Amuro Namie and Utada Hikaru) sales went down with time when she stopped being the big thing. But what has she, or have we as fans, lost?
She's rich as hell and continues to make loads of money from thousands of fans and she gets to make whatever the hell she wants, which will still be consumed and profitable thanks to those fans. She's doing great.
We as fans get regular new music, photo shoots, concerts, goods, just like we always have. We're doing great.
I get the disappointment, especially because jerks like the Arama writers and Japanese message board trolls want to tear her down and we want to show them up with a big success. But they don't matter. Ayu matters.
I enjoy the great stuff she's making.