By Lillian Rizzo and Joe Flint

For years, NBCUniversal's cable networks, channels like USA, Bravo and E!, were power centers in the media company, each with a mandate to promote their own programming and brands.

Those days are over.

As new Chief Executive Jeff Shell reshapes the entertainment giant to cope with cable TV cord-cutting and the rise of streaming video, he is centralizing decision-making -- from which shows get made to which networks those shows should run on -- and dramatically slimming down the cable unit in the process.

Top executives are getting squeezed out. Most recently, Chris McCumber, who led the USA and Syfy networks, said last week he was leaving after a two-decade run at the company. Substantial job cuts are expected throughout the cable entertainment group in the coming months, and some open positions will go unfilled, people familiar with the situation say.

The job of cable network president -- long a prestigious title in the entertainment industry -- is being phased out, a symbol of cable's shrinking importance. Mr. McCumber won't be replaced.

"The days of building that singular cable network with its own team and needs are long gone," said Mark Stern, a former Syfy executive and current president of Echoverse, a podcast studio.

NBCUniversal's top brass thinks certain entertainment channels don't have a long-term future on the cable dial, the people said. That includes E!, known for its red-carpet coverage and for giving the Kardashian family a platform; Syfy, home to sci-fi thrillers; and Oxygen, originally launched as a network for women that has lately pivoted to true-crime, the people said.

"NBCUniversal's cable networks carry some of the most popular programming in the industry and are enormously profitable," an NBC spokeswoman said. "They will continue to be a valuable part of our portfolio for fans, advertisers and our shareholders."

The cost-cutting could help persuade Wall Street that NBCUniversal, a unit of Comcast Corp., isn't over-investing in a declining medium. Comcast's businesses will likely be in sharp focus among investors after activist Trian Fund Management LP took a stake in the company, which The Wall Street Journal reported Monday. Trian believes Comcast shares are undervalued and has held talks with the cable company, but its views on specific areas of the business weren't clear.

NBCUniversal believes its brand power lies not in networks, but in individual franchises like "Keeping Up With the Kardashians," which airs on E! and is ending in 2021, and "Real Housewives" on Bravo, one of the people familiar with the situation said.

Mr. Shell and his newly appointed TV czar, Mark Lazarus, are putting a high priority on developing programs for the company's new streaming-video service, Peacock, which had 15 million sign-ups as of last week. Programming once intended only for broadcast or cable may also appear on Peacock, and Peacock original content may find a home on the networks.

"As long as it's being judged by the number of eyeballs, be happy wherever they come from," Mr. Stern said.

The future is also dimming for sports networks like the Golf Channel and NBC Sports Network. Hockey and soccer games are likely to appear more frequently on USA Network and Peacock, the people say.

The move to downsize cable networks comes as the pandemic weighs on NBCUniversal's business. Movie-theater closures hurt its film operation, its theme parks were closed and TV ad spending fell off. NBC's second-quarter revenue shrank 25% compared with the same period last year.

When Comcast acquired control of NBCUniversal nearly a decade ago, Chief Executive Brian Roberts cited the cable entertainment networks as a key attraction in the deal.

Such networks have been a cash cow for decades, because subscriber fees -- which cable providers pay to carry channels and then pass on to consumers through monthly bills -- have risen annually regardless of a channel's viewership. But cord-cutting has gradually picked up pace since 2014, and the pandemic is accelerating the trend for most operators, including Comcast.

The 2011 Comcast deal for NBCUniversal "was all about the cable networks, and the theme parks and movie studio were almost an afterthought," said analyst Craig Moffett, of MoffettNathanson. "Now, the cable networks are an albatross."

Several of NBCUniversal's entertainment networks, including Bravo, E!, Syfy and USA, have lost more than 10 million subscribers each since 2014, according to Nielsen data.

And providers are starting to push back hard on paying for unpopular channels. NBCUniversal could have difficult negotiations in coming months, including with its own parent, Comcast, as well as Charter Communications Inc. and Altice USA Inc., people familiar with the company's carriage agreements say. NBCUniversal's challenge is to keep enough fresh programming on each network to justify the cost to providers.

The cable networks are still profitable and have managed solid revenue and profit growth thanks to price increases. They accounted for $11.5 billion of NBCUniversal's nearly $34 billion in revenue last year, a decline from 2018 when the Winter Olympics took place. Comcast's primary growth engine, however, is clearly the high-speed internet business.

There are no immediate plans to sunset any well-known NBCUniversal channels, though some company insiders say they believe that is coming eventually. NBCUniversal has shut down other, albeit smaller, networks in the past, such as Esquire and Chiller in 2017, and G4 in 2014.

"One of the things they have to figure out is they have this collection of networks, and they have brands that exist in the traditional TV ecosystem, but which ones do you want to lean into in a digital world?" said Ben Swinburne, an analyst at Morgan Stanley.

NBCUniversal took steps in recent years, under Mr. Shell's predecessor, Steve Burke, to streamline some aspects of the cable entertainment group, centralizing some functions like finance and digital operations. Mr. Shell's proposed changes are even more extensive, bringing all programming decisions together for the first time -- not just for cable, but also Peacock and the flagship NBC broadcast network.

Mr. Lazarus, a longtime NBCUniversal executive, oversees the sprawling new TV and streaming segment. Frances Berwick, who previously oversaw E!, Bravo and Oxygen, was appointed to run the business operations across all of the entertainment properties. Former Warner Bros. executive Susan Rovner is slated to join NBCUniversal in coming weeks as the top creative executive.

Write to Lillian Rizzo at Lillian.Rizzo@wsj.com and Joe Flint at joe.flint@wsj.com