I think breaking things down into "alternative" and "mainstream" is not productive. I see no particular reason to come in with an
a priori assumption that the mainstream media should be given a default presumption of truthfulness until proven otherwise, and the alternative media should be presumed false unless proved otherwise. To me, they're all in the same category to start with, and each media source starts presumed false unless they prove themselves worthy of trust.
I mean, seriously... why should they be given a default presumption of truthfulness? Because they were... there? Because they were so honest before? But how do you know that? You can't just the honesty of a source if it's effectively the only source you have. (And in practice, despite there being "three news networks", they seem to have been the same source in practice. I can only remember minor sniping between the various sources every once in a while.) Why default to trusting them? I'd say it's basically just the availability heuristic at play, in a way. It's easy to see the availability of effectively a single source, and approximate that to honestly, but you have no rational basis to make that approximation.
If anything, after decades of fairly unquestioned and monolithic dominance, the simple heuristic of "unchecked power corrupts" suggests that the mainstream media is virtually 100% likely to be deeply corrupt. I find the idea that the media was allowed to operate in a state of highly ethical reporting for decades and it simply never occurred to any commercial, governmental, or intelligence entity that "Gee, golly, it sure would be useful to exert a lot of control over that" to be an idea that requires a degree of trust in commercial, governmental, and intelligence entities that I can find only marginal evidence in favor of and significant evidence against.
The media often actively seeks to manipulate and deceive, and provides biased and incredulous sources to do so, all the time.