I don't really agree or disagree, her voice, just like any other performers voice goes up and down - as it should naturally when performing, belting, screaming whatever you want to call it almost every night, it's just like working out - your voice is gonna feel the burn like your muscules will! The seasons are changing etc ... theres a lot of stuff that will take a toll on her voice, I wouldn't call it "downhill" at all though! For srue not downhill, cause it's never gotten progressivly worse.
She's still an encredibly talented performer, her voice needs a rest it's probably so tired from touring! (why am I talking about it like it's alive) but no really. Not downhill at all. She has her moments where she's not at her best but DONT WE ALL.