The Netflix offices in Los Angeles. (Lucy Nicholson/Reuters)

In 2011, Netflix executives began casting for a new show called “House of Cards,” gambling that high-end original television could be produced and distributed entirely outside traditional Hollywood.

Seven years later, that bet has turned into one of the great success stories in the history of entertainment. Netflix last week had more Emmy nominations than any other entity — 112 — from 40 different series. What had been a customer base of 20 million subscribers, mainly in the U.S., is now more than 120 million subscribers around the world. The company plans to spend more than $8 billion on content this year alone.

Yet in some respects Netflix’s future is more uncertain than it has been in a long time.

On Monday the company reported that in the previous three months it added 5 million global subscribers instead of the 6 million that forecasters expected. The news immediately prompted an after-hours sell-off, before the stock settled down 5 percent Tuesday at 380.

A shortfall of 1 million subscribers might seem minuscule to casual observers. But it touched on a number of concerns. With digital rivals like Apple jumping in to entertainment and entertainment foes like Disney pouncing on digital, Netflix’s fast-growing, big-spending ways suddenly don’t seem so ironclad. Investors are beginning to wonder whether a maverick giant is about to be disrupted itself.

“The reality is that if any traditional network would massively increase spending they would see a huge jump in their viewership too,” said Brian Wieser, a media-and-entertainment analyst at New York-based Pivotal, referring to Netflix. “Just because you’re spending a lot doesn’t mean you have a good business.”

Netflix at the moment remains the blue-chip giant of subscription streaming. The company generated $3.91 billion in revenue this quarter, and a profit of $291 million last quarter. The Emmy-nomination crown — the company took it from HBO, which held it for the previous 18 years — represented key symbolic victory. A ramp-up in originals in places like India and South America should keep subscriber growth brisk abroad even as it slows in the United States.

“We’re feeling very strong about the business,” Netflix CEO Reed Hastings said, speaking on a conference call with investors Monday.

The problem is that a whole new class of competitors feel that way, too.

In 2019 Disney will launch a service aimed at selling content directly to consumers. The company is close to completing its acquisition of 21st Century Fox that would give Disney access to units, such as FX and film division Fox Searchlight (“The Shape of Water”), tailor-made for the high-end world of streaming, in which prestige and engagement matter.

As part of the deal, Disney also would take over majority control of the streaming service Hulu, fresh off that company’s emerging hit “The Handmaids Tale” and strengthen its position against Netflix that way.

Meanwhile AT&T has been granted legal permission to buy Time Warner and in turn the chance to pour money into HBO to make the network an even stronger Netflix competitor. (The Justice Department recently appealed to try to stop the merger, but the deal already closed.)

The AT&T-appointed chief of the newly rebranded Warner Media, John Stankey, recently told HBO employees that he would like more “engagement.” That was interpreted in the room as a volume business that competes more with Netflix and its 40 Emmy-nominated series, according to one executive at the meeting who spoke on the condition of anonymity because they were not authorized to talk about it publicly,

The increased investment will almost certainly not approach $8 billion, said a person close to the situation who was not authorized to talk about it publicly, but will exceed the $1 billion the company is estimated to spend on content.

Tech companies and their deep pockets have also joined the fight to topple Netflix. Amazon’s streaming business has recently gone through a management change that has seen a number of NBC veterans, including Jennifer Salke and Vernon Sanders, take the helm. (Amazon’s chief executive, Jeffrey P. Bezos, owns The Washington Post.)

Apple has announced itself in a major way, in recent months signing up content creators including Oprah Winfrey and Steven Spielberg, whose shows it could distribute via an existing subscription platform such as Apple Music or a new service.

“Netflix once owned the future of television because they were the only ones doing it. And now in a way they’re in the weeds with everyone else,” said a Hollywood marketing executive who requested anonymity because his company both competes and does business with Netflix.

Netflix believes these competitors aren’t a direct threat. “The market for entertainment is so big that there can be multiple firms that are successful,” the company’s vice president of investor relations, Spencer Wang, told analysts on the call.

Still, observers point out that Netflix is in a uniquely vulnerable position: it doesn’t have another business if the subscription-content model falls off (or fall out of favor with Wall Street). That’s different from Disney, Apple and Warner Media, which all have chips on many hands.

A Netflix spokesman declined separate comment for this story.

Even analysts with a more bullish view on Netflix are taking a restrained approach toward the company. Though he believes Netflix has “taken a number of steps to forestall changes” and says the company has a bright future, Tuna Amobi, a senior equity analyst at CFRA Research, does worry about what he calls Netflix’s “entrenched incumbent status.”

“It’s definitely something we’re keeping an eye on in terms of the changing landscape — about 2-3 years from now the landscape is going to be more intense,” Amobi told The Post. “We’re already seeing that.”

One of the most potent illustrations that’s happening comes from a new study by cg42, a New York-based strategy and consulting firm. The company surveyed more than 3,000 TV viewers and found that cord-cutters pay for an average of 2.5 streaming video services, while so-called cord-nevers (who have never subscribed to cable or satellite), pay for an average of 1.5 services.

That suggests Netflix is well-positioned to keep charging customers since, among those surveyed, 93 percent of respondents said they had used Netflix in the last three months of 2016.

Except for one snag: In 2017, that number slid to 73 percent.

“It doesn’t mean Netflix has a problem as it relates to its business right now,” Stephen Beck, cg42’s founder, said. “It does mean the competitive environment is rapidly becoming more expansive.” If people only pay for a few services but there are twice as many of those services on offer, he said, Netflix could have to work that much harder to attract and retain subscribers.

That has been why, observers say, the company has been opening up its vaults for high-end creators. In the spring it agreed to a deal with prolific producer Ryan Murphy (“Glee,” “Pose”) that would reportedly pay Murphy as much as $300 million to bring his shows over from Fox. Shonda Rhimes, the ABC megaproducer behind shows like “Scandal” and “Gray’s Anatomy,” reportedly was paid as much as $100 million to join Netflix and has just moved her offices to the company’s Los Angeles headquarters. The company this spring also paid Barack and Michelle Obama a sum that wasn’t disclosed to produce shows and movies.

But simply paying a lot for talent may not be a sound strategy. “It’s the spaghetti theory,” said Pivotal’s Weiser. “Just spend money to see if something sticks.”

Netflix must also contend with its library of outside content shrinking. Disney and other giants are pulling shows and movies off Netflix as deals expire, hoping to monetize them in their own services. Netflix has known this day is coming, building up a deeper reservoir of titles as parallel services offer their own.

“The world is becoming more fragmented,” said the Hollywood marketing executive. “Netflix once benefited from that. Now it could be harmed by it. ”