Hmmmn, I think it's not easy to convince listeners with just simple melodies. Hikki and Adele are praised by their writings and vocals as well. I believe ayu's lyrics are really good (at least her older ones), but she's more about visualizations and rich arrangements. And honestly, ayu (and Madonna as well)'s vocals are not that great (if you compared them to Adele and hikki). It'll be much harder for ayu to convince listeners when she makes songs with simple melodies and arrangements.
And I think hikki has experienced such concept way before CWTC (tho I must say she has been heading to this direction since that moment). Dareka and BML were both done with simple melodies and arrangement. They were just, well, out of place in ULTRA BLUE
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