WHEN a TV show is declining in the ratings with little or no potential for improvement, it usually gets cancelled.

But “The CBS Evening News” isn’t just any ol’ TV show. Cancelling this particular TV show would be a tough thing to accomplish, even if it has all the earmarks of a show on the verge of annihilation. At 51 years, it’s CBS’s longest-running show. Moreover, it has the support of many powerful people within CBS – especially those who have devoted their working lives to CBS News – who feel it represents the heart of the company.

Some of those CBS execs may call to take issue with the first sentence of this column – that there’s little reason to believe that “The CBS Evening News” can reverse the declines that have it mired in third place.

The fact of the matter is that sometime in the next few years, one of the three network newscasts will have outlived its usefulness. The time will come when one of the three newscasts will be discontinued. That time isn’t now, but if it were, “The CBS Evening News” would be the most likely candidate.

There’s simply no evidence to suggest that the future will be bright for “The CBS Evening News.” Its aging audience is declining, and it’s not being replaced by younger viewers.

Moreover, news is so omnipresent on TV today that the network evening newscasts in general – and “The CBS Evening News” in particular – are irrelevant. Cable news channels, local station newscasts, financial news networks, and the Internet are all reporting the day’s big stories hours before you see them on the evening news.

Sooner or later, some exec at some network is going to successfully make the case that his network doesn’t need an evening newscast – that it can make more money with something else.

Unless things improve at “The CBS Evening News,” that scenario could become reality at CBS.