You know... that's a good question. If she does it in '17 then she could maybe revive her image/public interest enough for preparation for her 20th. But at the same time it seems it would fit so well for her 20th because... well... it's her 20th. xD
And, you're right, if it's done tastefully then it could definitely give her popularity a big boost. Namie's BEST FICTION was handled very skillfully. One, they released it at the perfect time. It was the time she was starting to change her image from a 'bad girl, don't touch me,' to a more 'cool girl, let's chill,' type of style which is more appealing to a wider audience (including more teenagers who, let's face it, are the general market for pop stars.) Two, her popularity was also starting to revive itself because of such releases like her singles Baby Don't Cry, Can't Sleep Can't Eat I'm Sick etc... leading into the release of her album, PLAY, which sold nearly 100,000 more than her previous album: Queen of Hip-Pop. So, since her popularity was already on the rise and she had a big grip on the general public, it was the perfect time to release a BEST album since they (as you put) sell like hotcakes (not just Ayu. BEST albums tend to sell better because... well... they have the artists best songs xD) So, yes, Namie's BEST album did revive her career, but that's because her career was already reviving.
Ayu, on the other hand, is kinda going down the other end. Her career is a bit stagnant right now. So releasing a BEST album could go either way. It could sell well, sure, but it may not do what it did for Namie in the long run since Namie was already climbing up the ladder, you know what I mean? But that doesn't mean it can't happen for Ayu. It just needs to be handled well.
... I just realized I didn't answer you question at all.
It just leaves me hoping that Ayu/Ayu's team handles this 20th very well. She's done waaaaay too much for the Japanese music scene to let it go by unnoticed. Hmm. Actually. No. 'Unnoticed' is not the term to use. There'll probably be lots promos (I'd hope). So I guess I should say, she's done too much for the Japanese music scene to let her 20th go by unappreciated.