I think an All Time Best album by Ayu would sell like 30k in the first day.
Namie has had 1 monster hit this decade (Love story) and many other hits like Hero, Mint, TSUKI, and so on; while Ayu's biggest hit this decade is progress, which was certified platinum.
By the way, people were telling this album price was cheap for containing 50+tracks? I think that was a very smart move.