In my opinion, the thing is, even though Ayu's popularity's dropping there's really no one to take her place...this was different for Namie a few years back because Ayu (and later Hikki maybe?) was right there to take her place as the reigning artist. So I guess unless Hikki makes a comeback or some new super talent comes along there's really no one to fill Ayu's spot, meaning she's still gonna stay on top!!
And I think you guys shouldn't worry bout the sales much...I mean, I would only get worried if Ayu's singles didn't reach no.1 or something, but since all of them did then that proves she's still quite popular. We may be used to hear Ayu getting no.1s but it's not that easy for other artists...
So yeh, no need to worry for now.~