"When exactly did the public become untrustworthy for music?"
If you look back to the Billboard charts of the decades past, some songs were forgettable, some songs were really slept on, but the ones that hit were legitimately seen as good songs from critics and fans, even to this day, obvious examples being Motown hits, girl group hits like "Be My Baby" and so on.
Then it seemed like there came a point where critics were heralding different songs than what the public liked, I don't know when this was (early 80s? I dunno)and this was when music snobs and 'guilty pleasures' grew and grew, to the point where now, if you want to be a true music fan, you would never tout liking the top 40 hits of Black Eyed Peas, or Rhianna and such, even if their choruses are sometimes better written than the erykah badus and Big KRITs of the world. I don't think thirty years from now cool critics will look kindly on Gaga and such like we look kindly on The Ronnettes or The Supremes. Why is that?
It's deeper now than not liking a Pop artist though. Us music snobs and such don't even know how these people can be so popular. Even the outsiders of yesteryear got why Michael Jackson was everywhere, rap heads that hated Cool J's "I need Love" understood why it was on the radio. But nowadays the popularity of Rhianna's "Birthday Girl" or Lil B's "I own Swag" is perplexing. The divide is so deep that even the pathology of stardom is lost on alot of folks. What song was the opening shot of this civil war?