On the other hand, the biggest, most respected of the mainstream publications, the NYT and the WaPo, have permanently tainted themselves by acting as stenographers for US government disinformation campaigns -- most notably in the run-up to the Iraq war, when they consistently promoted the fake WMD story.
From Morning Joe, this morning, you can already see that the New York Times has learnt lessons from that era: https://www.youtube.com/watch?v=OaI0u0ZdpMg
So the idea that they should still after all this time be permanently tainted is ridiculous and frankly is just an excuse by those who have different political persuasions.
Cokie Roberts is still employed, for example /snark
Sometimes it's Democracy Now!, a long-running counterculture mainstay that again presents an adversarial view and touches on subjects the mainstream media never would -- finding out about the Dakota Access Pipeline issue and reporting in depth months before the mainstream media deigned to grace it with even a moment of coverage.
Sometimes it's Breitbart. In those cases you are correct but 'alternative media' is an umbrella term that just means 'any group of people who do not behave as a mouthpiece for the political and economic elites'.
After the Telecommunications act of 1996 media has massively consolidated. At this point the 'mainstream media' refers to the media owned by those same 6 extremely powerful megacorporations that control all 'approved' 'serious' discussion. Alternative media means any alternate view.