Sometimes it's hard to judge the popularity of a song based upon airplay. As you know, nowadays executives in radio stations have the power to control what to play, and what not to. I'm not saying airplay is totally bu******* thing, but I can give an example right now --> Madonna's Hung Up. Was it popular back then? Yes. Did the radio stations play the song? Not really, cuz they still boycotting Madonna for the anti-war comments from last time.
By the way, Days gets no. 1 on overall weekly chart at recochoku this week. So I won't say Days is not popular at all..
http://recochoku.jp/recochoku_ranking/weekly_index.html
And about the airplay chart... You gotta explain how come MISIA's
CATCH THE RAINBOW is no. 1 all over the place when her single obviously doesn't sell that well. She's only no. 11 on this week oricon weekly chart, plus it is not even on ringtone/full song digital charts at all. I don't really know should we trust airplay (from radio station), or ringtone/full song/video clips downloads (from general public). You can see MISIA is
NO. 2 on Billboard Hot 100.. Somehow I find this ridiculous.