When did the mainstream media become part of the political system? It seems to me that the news media, whether mainstream or not, is separate from the political system.
There was such a time. It was back when journalism was taught as an honorable craft in universities. Just as stonemasons and bricklayers took pride in laying straight and strong walls, honoring their craft, journalists took pride in checking facts, reporting on controversies from all the sides they could, explaining arguments rather than advancing them. Remaining outside the argument. Believing in the possibility of objectivity. Newspapers that took sides in the reporting of facts were scorned as "yellow journalism" for being unfaithful to the craft. Journalists didn't always live up to the ideal, but at least there was an ideal of objective reporting, an ethic of accuracy and fairness. The readers and listeners were entitled to that.
That ethic is now considered quaint. The concept is ridiculed, especially by movement conservatives.
I think people watch and listen to political news outlets now for the same reasons people used to buy dime novels about Wild Bill Hickock and the Wild west. The stories are loosely based on reality, sort of like reality, but much more exciting. People get to participate not as citizens of the country, but as fans of the characters, and it's a lot more fun.
It's still yellow journalism to me.