Jon Stewart's departure after 16 years at "The Daily Show" hits an already struggling Viacom.
Stewart helped turn the company's Comedy Central into a TV force, and now it doesn't have any natural successors for him. Last year, it lost Stephen Colbert, as well as John Oliver.
Read MoreJon Stewart is leaving 'The Daily Show'
The departure of Stewart, who didn't announce his plans, fuels investor concerns that carriers will drop the channel and advertiser interest will plummet further.
This past quarter, Viacom's ratings slump dragged domestic advertising revenue down 6 percent. This decline comes as ratings have dropped at MTV and Comedy Central, which saw ratings in the key 18-49-year-old demographic decline more than 10 percent.
Even Stewart's ratings have declined this year, thanks to new competition from Jimmy Fallon's "The Tonight Show." This year, Stewart's viewers in the most valuable demographic—18-34-year-olds—has dropped by 22 percent. It's still larger than Fallon's, though NBC's "Tonight Show" is making gains. (Disclosure: NBCUniversal is the parent of NBC, CNBC and CNBC.com.)
"Jon Stewart was one of the must-buys for advertisers in the Viacom cable network portfolio—people actually watched it live," said BTIG analyst Rich Greenfield. "So this adds risk to both subscriber losses and advertising; remember advertising is already declining single digits."
Greenfield downgraded Viacom shares last June on concerns that Viacom didn't have the must-see TV to secure advertising increases and to guarantee its channels would continue to be included in pay TV bundles and would be part of new over-the-top streaming services like Dish Network's SlingTV.
"Despite its challenges, fake news has proven itself a profitable format for Comedy Central, as well as an important strategic tool for Viacom, given how much talent has been on and moved on from 'The Daily Show,'" said Wedbush analyst James Dix. "Given Jon Stewart's long tenure and upcoming contract, management is unlikely to be completely surprised by his decision to retire. Let's hope the company can come up with a solution that builds some interest among younger demographics for the 2016 election votes and advertising dollars hang in the balance."
While Comedy Central generates less than 10 percent of Viacom's annual revenue, the channel's impact is much larger, said Wunderlich Securities' Matt Harrigan. "Comedy Central is perhaps $700mm in sales out of over $10bn for the last fiscal year for the MTV Networks group, but this is still a perceptual negative given his huge profile and ratings issues at certain of the networks."