Roger Mudd returned from vacation the other day to find that as of Sept. 5 he is to be "disappeared" as co-anchor of the "NBC Nightly News." In a business whose impresarios are celebrated for boneheadedness, Mudd's demotion wins the brass ring.

If people who know and work in government could choose their executioners, they would choose Mudd hands down. He keeps an undeceived eye on the politicians, but the eye is tempered by mercy; and when he conducts one of his gentle decapitations, all the victim feels is that "refreshing coolness at the back of the neck" that Dr. Guillotine promised the users of his instrument.

Mudd's removal is said to be, as usual, the result of mindless obeisance to the "ratings." Its sweatier critics charge television with bias. But bias, though it exists, is the least of TV's threats to the republic.

One of the greatest, of which Mudd's demotion is symptomatic, is what The New Yorker's TV critic, Michael Arlen, calls "the institutionalization of soft news"--a continuing reduction in serious coverage of government (not politics--there is a difference).

My own survey, as an irregular but attentive viewer of evening news programs, shows a progressive sacrifice of news values to the virtuosity of the color camera. Fires, floods, earthquakes, volcanic eruptions, lava flows, California mudslides--these horsemen of the modern suburban apocalypse occupy more and more of the nightly 24 minutes of news.

They make better pictures than a budget deficit or the sale of a deadly missile to the belligerent in some regional civil war. So natural disasters, along with staged political events and alleged ecological menaces, steadily crowd government--its personalities, institutions and processes--off the air.

By giving Roger Mudd equal billing with Tom Brokaw, NBC had at least seemed to keep a window on government ajar. And with Mudd, it was a sophisticated window. Once Mudd moves to the wings as a "senior political correspondent," you will hear the window quietly begin to close. And those who say they depend on network news programs for the "news" will get an even more skewed, and often incoherent, view of the world.

Incoherent, did I say? My own example is the way the continuing chemical-waste and nuclear- power stories are dealt with. Night after night, we follow the EPA men in their rubber suits and deep-sea masks as they explore some small town where a dioxin scare has been raised.

The story, typically, is only that traces of the chemical are there, not that there is considerable uncertainty that dioxin in low concentrations is really the public health menace implied. It's probably the most overbeaten public health horse going; but it has become, with volcanoes, a network favorite.

Nuclear power plays much the same way--as a hazard pure and simple. The statistical risks, even after Three Mile Island, may be lower than those of being run over by a tricycle. But cooling towers make menacing film footage, as does the confusion of those who live in their shadow. Power dim- outs and brownouts, the environmental hazards of coal-fired generators, do not make comparable film.

Every time some incident excites gloomy reflections on the drift of television news styles, one is reminded that styles change. You have only to see one of those pre-television movie newsreels, with its hearty, booming voice-over and air of indoctrination to see how visual modes have improved. Even the classic public-affairs television of the Edward R. Murrow age now looks, to our eyes, stilted, technically primitive and "hot," for all its appealing earnestness.

The trend--from hot to cool, from earnest to detached, from rough to smooth, and by and large with the salesmanship of a viewpoint much modulated--is largely positive.

The question is what network television will do with its growing technical virtuosity. Roger Mudd's demotion by the ratings-obsessed people at NBC suggests that the ratings rat race will ultimately trivialize the news, as it has trivialized commercial television entertainment.