You don't think journalists want to get/keep jobs? You don't think editors want their newspapers to be influential and financially successful?Of course they do. And the way they learned to do that is to always meet their deadlines, to select stories based on emotional impact, "if it bleeds, it leads." I am saying that what makes a newspaper successful is what drives a newspaper to be liberal. Only "liberal" isn't the right word. It's the exact same attitude, but in a journalist it's called "objective."
I am saying that successful newspapers are shallow, negative to the point of cynicism, and arrogant. Looked at it from an economic standpoint you can understand why it would be that way - and looked at from a political perspective, it is leftism. Because all leftism is is second guessing the people who are responsible to get things done. People who actually do things can always be second guessed, and leftism is nothing but second guessing. That's why when a liberal like x42 got into executive office he didn't do the job of president, he simply kept doing the job he was good at - which was running for president. Permanent campaign.
The operative word in my first sentence you missed is "just", as in yes, they are concerned about job security but it isn't their only agenda item by a long shot.
I agree with most of the rest of your post, but I think you're over-thinking the entire matter. Yes, they want to keep their jobs but the leftism isn't as abstract nor as necessarily tied as closely to the process of journalism as you state. For one thing, journalism has been around for a very long time. It's been in this country since we landed at Plymouth Rock, basically, but it only became a strong instrument of the left in the last fifty or so years.
At any rate, I subscribe more to the Fifth Column theory, which is really a cart-before-the-horse argument in terms of our discussion. I think leftists seek out journalism careers because they have an agenda to undermine and bend this country to their will and their world view, rather than them conforming to the demands of the industry as being the sole reason for journalism being leftwing.