However, the credibility source of a factual claim does have a bearing on whether that factual claim is more or less likely to be true. Which is why you're allowed to impeach witnesses in a criminal trial. Whether someone has a history of lying doesn't affect the veracity of them arguing that "2+2=4" (which doesn't rely on their witnessing it to be demonstrated), but it would affect the veracity of them claiming that they saw me punch a kitten.

Which is a legal tactic meant to affect the perception of the jury towards the witness. It says nothing about the veracity of the claims of the witness.

If a known liar says 2+2=4, that's a true statement. Somebody on a jury who doesn't know that adding 2 and 2 to get 4 would be prejudiced against that witness, however (which is why that tactic is used).

Back when there were only three news stations, the commercial success of those news stations was more directly linked to the credibility and reliability of their reporting. Because they had to appeal to an audience that included nearly everyone, all of their economic incentives drove them towards garnering a reputation for investigative accuracy.

Or, looking at it from the other perspective, people who think about things in identical ways tend to report the same things with the same biases and the same lenses. Hence Walter Cronkite's reporting not being much different from John Chancellor's, say. Was Cronkite a straight deliverer of the news or did he use his platform to advance his opinion?

Without a splintered audience, the only way for news outlets to differentiate themselves and gain (or lose) market share was the quality of their reportage - because a desire for accuracy was the only common denominator in the demand of such a diverse customer base. It's one of the few things that everyone in the audience wants. There was no way to make money by narrowcasting untruths in broadcast "over-the-air" television - those untruths would destroy you with the rest of the audience.

This doesn't hold up either. Witness Dateline NBC: latimes.com - La xpm mn story
Doing sensational things for ratings has been a feature of media for decades.

You do. For yourself. But for that to work, you have to make the effort to actually figure out whether a source is in fact trying to search for truth or to support a particular "side."

That's exactly the point I'm making: One side of posters here can't claim victory by just dumping on something they don't like to read.