“ Most educated people believe that premier news outlets are inherently trustworthy.” Let’s stop right there. I think the last 8 years have shown the media to have lost significant amounts of public trust. I would say that most educated people inherently believe that the media should not automatically receive benefit of the doubt any longer.
What happened in the last 8 years? In 2003 there was the war in Iraq. The fallout from that was probably the lowest trust I had in journalism at any point in my life. I can't recall any significant events in the past 8 years that are even close to that failure.
He did two major things.
First, he relentlessly attacked the media ("fake news") and regularly called them out on false or misleading stories. (Then the media regularly complained about him eroding their credibility, without admitting that his criticisms were often valid.)
Second, he was incredibly good at manipulation. His signature move was to tell a big fat lie about an issue that he wanted everybody talking about and where his general position was a popular one. The media hated this, because their choices were to let him get away with the lie or to start a conversation about the issue Trump wanted everybody talking about. But then they let their hatred of him become so transparent in their coverage that it undermined their credibility.