The evidence is that the causal arrow points the other way: The political movement attacks quality journalism - for example attacking reporting on election results, climate change, vaccines, and much more - and promotes disinformation.
That doesn't mean there isn't degredation in quality journalism, but I see no evidence that it's a cause of the reduction in trust. The reduction in trust is greatest in the political movement described above.
I would argue that the degradation of journalism came first, and what allowed such political movements. An intertwined positive feedback loop to be sure, but i think its the weakening of journalism that allowed such movements to get a toe hold to start with.
Unless they mix together or there are other causes involved.
First, when was journalism 'weakened'? The conservative anti-journalism campaign (to be clear, not all conservatives are anti-journalism) goes back to people like Rush Limbaugh in the late 1980s and Fox News in the mid-1990s. And journalism lost funding due to Craigslist taking over the main source of income, classified ads.
Second, if these people are really interested in good journalism and truth, why do they attack journalists for reporting truth and insist on disinformation?
Maybe come up with some evidence of your own, for your own argument. Mine won't help you much.
Since you decline to present any, it's clear you have none. Why not look up the evidence and see what you can learn? It might be different than what I percieve, and at least you'll be learning.
That doesn't mean there isn't degredation in quality journalism, but I see no evidence that it's a cause of the reduction in trust. The reduction in trust is greatest in the political movement described above.