Ayaka has a strong voice, maybe stronger than Hikki, so she might have been able to gain more popularity by releasing a song now while Hikki is busy in the States.
Of course, no one can really replace Hikki but it would have been a good strategic move. Alas, she just got married and probably won't be releasing any new songs for awhile.
Well, here's one of her old songs that has over 1.5 million hits.
[youtube]qSobb9w_4JE&feature=related[/youtube]