You know why it's mainstream? Because it's nuanced, and well-researched and run by actual real-life journalists.
You know what you're left with when the media is no longer mainstream? The extremes.
American news consumers should be critical, and pay attention to times when major competing news outlets are running the same headline about a complicated story.
Which "Mainstream media" outlets do you consider to be nuanced and well-researched?
I would put the opposite question, which non-mainstream media outlets would you consider to be nuanced and well-researched?
Ahem.
“Briefly stated, the Gell-Mann Amnesia effect is as follows. You open the newspaper to an article on some subject you know well. In Murray's case, physics. In mine, show business. You read the article and see the journalist has absolutely no understanding of either the facts or the issues. Often, the article is so wrong it actually presents the story backward—reversing cause and effect. I call these the "wet streets cause rain" stories. Paper's full of them.
"In any case, you read with exasperation or amusement the multiple errors in a story, and then turn the page to national or international affairs, and read as if the rest of the newspaper was somehow more accurate about Palestine than the baloney you just read. You turn the page, and forget what you know.”
― Michael Crichton
1. It assumes that inexperience in reporting correlates well across disciplines (i.e. if you read one badly researched physics article, it means the sports pages are similarly bullshit)
2. It assumes the existence of better-informed alternative media
In the particular case Crichton cites, it's entirely reasonable to assume a news outfit might have better reporting on Palestine then on physics, if only because politicians have an incentive to talk to news outfits where scientists might not. In fact, much of journalistic ethics and process revolves around trying to determine political "truth", as much as something like that can exist. Conversely, there are plenty of scientists who can explain the details of the Standard Model or Quantum Field Theory; but don't have a single clue as to how trade negotiations work or why we sided with Saudi Arabia despite them not sharing our values one bit.
Michael Crichton assumes knowledge is an RPG stat and that if a newspaper fails a knowledge roll once we should discard the paper because it's writers didn't put enough points into INT.
Second, most alternative media I've encountered does no better on the "wet streets cause rain" problem. Actually, my experience is that they're more prone to shoddy reporting, because their goal isn't to try and find out something approaching the truth. It's to sell me a series of plausible lies that lead up to their particular political bias.
For example, let's talk about climate science. It's a demonstrable fact that the Earth is warming due to human activity, but there's multiple ways one can be wrong about that. The kind of inaccurate reporting I'd expect to see out of a mainstream outfit would be something like "CO2 causes ozone depletion". A right-wing alternative rag would instead try to deny the science altogether, either by casting FUD on the scientific process or worse. The former is something that might be technically incorrect, but the latter is deliberately trying to convince me that black is white, up is down, and short is long.