It's clear that if CBS (CBS - Get Report) and Viacom (VIAB - Get Report) didn't have the same controlling shareholder, few people would say these two companies ought to merge.

For 13 of the past 14 years, CBS has led the ratings war because it focuses on an older demographic that still watches television. Chief executive Leslie Moonves and his creative team certainly deserve mountains of credit for consistently pumping out hits -- NCIS, Big Bang Theory, Criminal Minds, Survivor -- the list goes on.

But while CBS's audience historically has skewed older than those of its rivals, the network Moonves never tires of saying is the country's "most watched" made a smart bet: produce programming that appeals to as wide an audience as possible. The network actually won the 2015-16 season in the highly coveted advertiser demographic of 18-to-49-year-olds, according to Nielsen -- only the third time since record-keeping began in the 1980s.

So, it's hardly fair game to call CBS the geriatrics network.

As for Viacom, its 26 cable networks led by MTV and Nickelodeon are in the midst of a steady audience and ad sales decline, a victim of focusing on a demographic that has long since turned to a myriad of online options from Alphabet's (GOOG - Get Report) YouTube to Refinery29, the women-focused website in which Time Warner (TWX) invested $45 million in August.

Viacom this year is expected to post its lowest annual revenue since 2006.

Nonetheless, trustees at these two companies have been asked by their controlling shareholder, Sumner Redstone's National Amusements, to consider merging the two largest pieces of a $40 billion media empire increasing overseen by his daughter, Shari Redstone.

"The whole CBS story is dependent on having great content, and if you start to lose that by having people distracted by having to manage weaker networks, you risk killing the golden goose in the process," said Tony Wible, a media analyst at Drexel Hamilton. "This is absolutely great for Viacom shareholders, but I'm not sure it's in the best interests of CBS."

For all of the accolades about Moonves' leadership, CBS isn't without longer-term concerns. Ad sales, which accounted for 47% of CBS's revenue in the second quarter, dropped 2.6% to $1.55 billion. And even the network's stellar programming streak isn't without its blemishes -- Limitless, Angel From Hell and The McCarthys proved that not everything Moonves touches turns to gold. 

Viewership of National Football League games is also down. Audiences for games of the first four weeks of the 2016-17 NFL season trailed viewing from a year ago, according to Nielsen's metered-market rating. 

Taken together, ratings for NFL games broadcast on CBS as well as 21st Century Fox  (FOXA) , Disney's (DIS - Get Report) ABC and Comcast's (CMCSA - Get Report) NBC were 10% lower for the first week of the NFL season, 12% behind those of the second week and 11% less than games televised during those same weeks in 2015. CBS's regional NFL coverage on Oct. 2 yielded ratings 2% lower than a year ago, according to Sports Media Watch

The culprit seems to be a combination of less interest from younger viewers, the lack of outstanding teams and negative publicity around issues such as concussions, domestic violence and the national anthem. 

In a Sept. 29 investor note, Jefferies media analyst John Janedis was quick to underscore that these are early events in a long NFL season. Yet the importance of the NFL to major media companies can't be overstated.

CBS, which spends about $45 million for each Thursday night game, generates about $615 million a year from televising the NFL, roughly 12% of its total ad sales. Televising NFL games is also integral to publicizing its fall primetime lineup as well as the late-night talk shows of Stephen Colbert and James Corden and long-running staples such as 60 Minutes. CBS will televise 57 hours of NFL programming in the fourth quarter, second only to NBC at 60 hours.

"While it may be too early to call a trend, given the significant amount of revenue and expense associated with sports programming, it is something worth keeping an eye on," Janedis said.

Which brings us back to Viacom. 

Whether standalone or re-merged into CBS, fixing Viacom's cable TV networks will require spending a lot of money on new programming and the creative minds to give a new shine to its core networks: MTV, Nickelodeon, VH1, Comedy Central and BET. Operating income at Viacom's cable networks in the June quarter was $872 million, its lowest for the three-month period since 2009.

Making matter worse, programming costs at Viacom's networks are already above the industry average, according to Goldman Sachs media analyst Drew Borst. Throwing good money after bad hasn't put a stop to the slide, Borst said, "suggesting a potentially more deep-rooted problem driven by a secular decline in TV viewing for the kids, teens, and Millennials that Viacom targets."

Yes, a combined CBS-Viacom would save overhead costs and boost net income, but higher execution risk and leverage threatens to weaken CBS's growth prospects. Turning around Viacom's biggest brands may require not just better programming but appealing to an audience that is already doing its own aggregating rather than looking to a network to do it for them.

"Unless CBS has a time machine, there's nothing they can do to get kids and teens to go back to watching scheduled, ad-supported linear TV," Bernstein media analyst Todd Juenger said in Oct. 5 investor note. "Even if CBS succeeds in revitalizing the Viacom brands in an on-demand world (very difficult), the resulting economics (and value) are vastly inferior to Viacom's old business model."

The only path to a merger may be allowing CBS to acquire Viacom at a discount to its current share price. CBS also would want the flexibility to combine or close any number of its 26 networks. At present, 85% of Viacom's cable TV earnings comes from its six largest channels.

Moonves also may see the benefits of owning a Hollywood studio. Viacom's Paramount, which lost direction under former CEO Philippe Dauman, is a prized asset in a media industry that became global some years ago.

Moonves isn't a stranger to the movie business. His relatively small production house CBS Films developed Patriots Day, based on a 60 Minutes piece. The film is scheduled to be released in December in a distribution deal with Lions Gate Entertainment (LGF) . There are only a handful of major Hollywood studios, so owning Paramount would give Moonves a valuable platform.

But CBS's strength remains its ability to produce programming that resonates with mainstream audiences and sell that programming to some 200 broadcast affiliates, the company's 30 U.S. TV stations as well as pay-TV providers in the U.S. and abroad. CBS is also hedging against the decline in pay-TV subscribers through its online platforms CBS All-Access and Showtime Now, which have attracted 2 million subscribers since they were launched in 2014.

So does CBS really need Viacom?

"It's a risky deal," Wible said. "If Les leaves the company or is distracted by a merger, does the whole thing fall apart? CBS is totally dependent on them having hit programming."

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