I definitely feel like her creativity doesn't excite me the way it used to, but honestly, my interest in her music decreasing has a lot more to do with my waning interest in japanese culture and music in general. Around the time I became a fan (2003), my weeabo levels were too damn high
But even so, her music was still very exciting, different in arrangement compared to popular western music and even jpop at that time, and I hardly listened to music, because it was all the same old. Today, I almost feel like it's the other way around - the original and exciting songs in jpop are getting fewer and fewer, but I keep getting introduced to really amazing western music.
So, it's a combination of many things. Ayu does pretty much the same thing over and over (and why wouldn't she? It works for her, and her music is decent. And well, with the exception of a few songs where she still manages to surprise me), I don't have the enormous levels of interest in japanese music I used to, and western music just is overall better, I feel. I can't really bash Ayu, musically, she's had few misses and a whole lot of outstanding songs.