Just one thing which I feel: watch past lives, it seems it's really easy for her to sing every notes, with a clear and controlled voice. Now she is literally struggling to sing properly.
Just one
random 'old' live from youtube:
Compare it to You were... lives (i think those songs are quite similar to sing) and the gap is pretty obvious isn't it?
I think that her last lives where she seems to sing well without much effort were in 2007 (A BEST 2).
It's worsening in the sense that she can sing properly nowadays.