One could argue that the networks are making up for their loss in viewership by charging advertisers more per viewer — but that’s not how they see it.

The logic: Because audiences are more fragmented than they once were, there are fewer places to reach large groups of people all at once. In the current cultural climate, programs that millions prefer to watch in real time, in campfire moments of togetherness, have become rare and especially valuable. A viral YouTube video may generate billions of views, but they don’t occur at the same time.

The ad industry is still big on 30-second commercials meant to appeal to the broadest possible swath of consumers. Such mass-appeal ads are less potent on the internet, which allows marketers to target people whose spending habits are tracked purchase by purchase. Advertisers are also wary of the internet’s unregulated spaces, which can place a brand’s message next to videos featuring hate speech.

“If you look at it in those terms, a brand-safe environment, with no fraud and a guaranteed audience in the demo — maybe TV is a little bit underpriced,” said Mr. Schwartz of GroupM.

And yet the internet has something TV is losing: the young people that advertisers crave. By emphasizing their appeal to the youth market, technology giants like YouTube and Instagram have become adept at stealing a portion of ad budgets that once went to the networks.

YouTube has assembled a roster of homegrown stars who are vetted and safe for advertising, creating the equivalent of TV channels known to media buyers as Google Preferred. The streaming site showcased its stars at a high-gloss event at Radio City this month to an audience of ad executives and fans. Bartenders in the lobby checked wristbands to avoid serving drinks to those who were underage, something that won’t be a problem at the upfronts.

Wait. Does TV even matter anymore?

One word: sports.

There are other reasons that old-school TV is still relevant, like higher production values and the chance of attracting a mass, real-time audiences. But sports programming may be the main driver of viewership now, and it has slowed the cord-cutting trend.