But a profound shift has begun to seize the television industry: Ratings no longer hold a key to a show’s success.

The traditional criterion for the renewal of a series has been supplemented by other factors, including networks’ ability to sell the shows to overseas and streaming platforms. What had long been regarded as a kind of brutal meritocracy — only the most popular shows, which drive the most ad sales, are allowed to survive — is now deeply influenced by other variables, leading some shows to stay on the air for years on end with very few viewers.

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“Broadcast television used to be about ratings,” said Preston Beckman, a longtime Fox and NBC television executive who helped shepherd the latter’s must-see TV schedule of the 1990s. “It’s become about a lot more complex factors.”

Ratings, tabulated by Nielsen to include DVR views on the same day, can be evaluated in various ways. But shows whose episodes regularly draw under 5 million viewers are often seen as struggling, especially if they’re not disproportionately strong in the 18-to-49 demographic that advertisers care about.

On one hand, the longevity of these little-seen shows affords them more time to find viewers and a creative footing. But some cultural experts also say it leads to the blocking of fresher ideas and deepens consumer fatigue with broadcast networks, already lacking cachet in the streaming age.

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Examples of such resilience abound. A dying TV show in 2018 doesn’t keel over as much as wander through an endless night like a character on “iZombie” (the CW, 1.2 million average total viewers last season, now entering its fifth season) or cling stubbornly to life like an organism under Mr. Griffin’s microscope from “A.P. Bio” (NBC, just 2.8 million average total viewers, proudly entering its second season).

In fact, last year marked the first time since the ratings site TV By The Numbers began tracking figures nearly a decade ago that fewer than half of networks’ first-year series were canceled. That marks a severe drop-off — the number once topped 70 percent.

And as the fall season begins this month, 13 shows are entering at least their 10th season, believed to be a modern-day record. That includes such programs as “Grey’s Anatomy,” entering Season 15, and “The Simpsons,” entering Season 30. Viewership for each of these shows is down more than 70 percent from their all-time highs.

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Even series that do finally seem dead can earn a second life when another network comes along with a defibrillator, as happened with “Brooklyn Nine-Nine” (Fox, an average of 1.5 million viewers in its most recent season). After the network canceled the show in the spring, NBC swooped in to pick it up.

The shifts are being driven by multiple factors. Broadcast shows overall draw lower numbers in this age, as the generational drift away from broadcast TV to cable and services such as Netflix and Hulu has greatly reduced network viewership.

“In this climate, if you’re not going down, you’re going up,” said Matt Berry, a longtime producer of shows such as “Desperate Housewives” and Tim Allen’s “Last Man Standing,” which was picked up this season by Fox after ABC canceled it in 2017.

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For many directors, that ratings ceiling has led executives to wonder why they’re spending millions launching new shows whose numbers might only match ones already on the air. Broadcasters also have to compete with more outlets to find quality new shows in the first place.

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But even within the lower expectations, some surviving shows stand out for their poor performance. They remain on the air, however, because networks are looking beyond advertisers to make their money. The market for licensing series after they air — particularly overseas but also on streamers and other platforms — is booming, so many networks keep a show afloat even if traditional ratings are weak.

That is, assuming the show is produced by a network’s studio, allowing the parent corporation to collect the profit. Shows produced in-house increasingly hold advantages over shows produced by outside studios.

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“Taken” (NBC, a first-season average of 4 million) was shown the kind of mercy hit man Bryan Mills would never display when NBC renewed it last year. That’s because NBC owned the series, which held international appeal, and also split costs with its European producer. Ratings continued to sink in the second season, in some weeks barely reaching 2 million total viewers. NBC finally put it out of its misery in June.

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The network made a similar move with Fox’s “Brooklyn Nine-Nine” because it’s produced by NBC’s own Universal Television.

“Ever since we sold this show to Fox I’ve regretted letting it get away, and it’s high time it came back to its rightful home,” NBC Entertainment Chairman Robert Greenblatt said after the pickup.

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Such a long leash means an opportunity for new work to take chances without fear of a quick cancellation, a development as welcome as a shrewd custody arrangement on “Splitting Up Together” (ABC, 4.2 million viewers but renewed for a second season beginning next month) or a Rebecca a cappella number on “Crazy Ex-Girlfriend” (CW, just 800,000 viewers in its second season, now entering its fourth).

But keeping older series hanging on more tenuously than an Elizabeth McCord peace deal on “Madam Secretary” (CBS, entering its fifth season despite a drop of nearly 50 percent in total viewers last year) comes with a significant downside.

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“We’re in a time when it’s much easier to stay on the air but much harder to get on the air,” said Danny Strong, the actor-writer behind “Empire” who clawed his way up a long list of pilots to get his new legal drama “Proven Innocent” a midseason deal this season on Fox. “That’s either very good or very bad for television, depending on who you are.”

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Strong echoes what many creators feel — that the never-ending series means fewer slots for their more original shows.

While it’s impossible to know why a given show doesn’t make the cut, the trend to longevity has appeared to edge out some bolder ideas. This year, a fresh take on “L.A. Confidential” (CBS); Katie Holmes’s return to television as an FBI agent (Fox); and a high school reunion comedy from Oscar-winning screenwriter Diablo Cody (ABC) were all among the promising pilots that failed to get pickups.

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“The cultural consequence here that with all these ‘NCIS’ and ‘Law & Orders’ hanging on, you’re left to wonder about the innovation that could be happening instead,” said Robert Thompson, a television historian and expert at Syracuse University. “It’s Darwinian: With all these old stodgy organisms, there’s no space for new mutations.”

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The trend also reinforces the perception that networks are musty places missing out on all the action. “Broadcast is not where a lot of young viewers want to be, and I think some of these [older] shows are the reason why,” said Kevin Sandler, a professor at Arizona State University who specializes in TV and media.

TV insiders acknowledge the platform can be less than dynamic. But they say that isn’t necessarily a bad thing.

“People say broadcast television is a place for creative bankruptcy,” said Peter Lenkov, a veteran writer-producer who has served on long-running shows such as “CSI: NY” and reboots such as “Magnum P.I.,” which debuted Monday. “But I think that’s not the right way to look at it. I look at TV less as a place of discovery as a continuation. It’s a place you can keep coming back to the characters you love.”

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In the current climate, networks can also have a harder time pulling the plug because it’s more likely they’ll hear from fans when they do.

What was once the rarest of events — a fan campaign, carried out via handwritten letters, like the pioneering mission to save the 1980s police drama “Cagney & Lacey” early in its run — is now commonplace.

Such was the case with “Timeless,” the time-travel series through American history whose ratings were bloodier than Gettysburg (1.8 million viewers in the 18-to-49 demographic after the second season ended in spring).

But audiences mounted a social-media campaign, giving producer Sony Pictures Television leverage to negotiate a deal with NBC for a stand-alone movie later this year. Sony is now contemplating making a “Timeless” return an annual event, according to a person close to the show who was not authorized to talk about it publicly.