I don't have the Japanese perspective since...I don't live there, but generally what makes an artist legendary is influence rather than success itself.
AKB48 may sell a lot, but nothing they do has a lasting influence on the music business or the general public because what they do is relatively generic(I am NOT saying it is bad) and many girlgroups before have done similar things. They also have one core fanbase that is responsible for the high amount of sales, especially by buying multiple copies each as that is encouraged by the label, rather than having sales driven by casual listeners, visible by the first day sales, concert attendences...
Ayu, however, had lasting influences on music, other aspects of Pop(visuals, lyrical themes...) and the general public - and all that while selling a higher overall number of units than anybody else(as I said, success alone does not make you legendary, but it certainly helps, if Madonna had sold only 5 million albums she would not be considered legendary). To add to that, Ayumi, being a solo artist that always focused on her lyrics and personal expression, has much more of an "identity" then the AKB48 girls, who are essentially all replaceable as the artistic input doesn't come from them. You need to have an unique identity to be legendary, else there is nothing to truly associate the legacy with.
I hope this helped
