If you said ayu's music is "STALE", then you try to listen BoA or Mika, then you will find out what STALE is truly mean.
actually, there are only little variety music in the world, what ayu can try is very
limited, she have try almost everyone(except R&B), if you find that ayu is stale, then listen to other artists who mostly have NO variety in there music. such as i find that mika's music is absolutely boring, all the songs are the same(IMO). that kind of music will let their fans gone away because songs are always the SAME.
hikiki is also(imo), her deep river album's song is absolutely repeated, i find that the arrangement of the whole ablum is almost the same(esp. tokyo night,***i love you). that kind of songs are called stale.(BoA ,mika and some bands (not to name it)are truly stale , their music lacks variety.
but i agree ayu's song is going popish
,the main reason is due to the arrager H^L, one of the member of H^L(the best one) had left the team, since then, their music quality have move A LOT backward 
poor ayu