"Assured" may have been a strong word, but they really had to screw up to not make a ton of money. And one of the major reasons they were set up to make all this money was the deal they made with the public - the airwaves in exchange for programming in the public interest. What I'm arguing is that they actually took their end of this bargain seriously for many years. Of course, they wanted to make as much money as possible, but CBS saw the news division as primarily fulfilling this role of serving the public interest, in exchange for the opportunity to make loads of money off the public airwaves in myriad other ways*.
Look, I'm not trying to suggest that the corporate magnates at CBS, ABC, etc., were any less interested in profit than any other business leaders. What I am suggesting is that they had an appreciation that their profit came from the deal they made with the public, and the news divisions were how they fulfilled their end of the deal, and that was why the news divisions were able to operate relatively independently for many years.
I think this probably rubbed off (in a good way) on print journalism. WaPo and NYT couldn't be engaging in Hearstian shenanigans, because it damaged their prestige, and the role of the networks as protectors of the public good played a big role in that. And, yes, of course, the public response. As the public came to expect quality, objective journalism, then smart business dictated that media outlets give it to them. So it was doubly-reinforcing. Not only did we have people who felt some sense of a duty to "do it the right way," so to speak, but doing it the right way, for a very long time, was conducive to increased readership/viewership - and thus, profits.
Now, of course, precious few of the public expect that kind of journalism, and even fewer can be trusted to routinely consume it. So that end of the doubly-reinforcing mechanism is incredibly weak. And for the reasons I outlined in the previous post, the other end - the "duty" end - is also at a low point. So we get what we have now.
* Or, "a myriad of other ways," for those so ridiculously inclined.