The Marvels made about $47 million this weekend, a disappointing number that missed already lowered expectations of $60 million that analysts predicted earlier this month.

A major reason for the film’s flop is that the workload placed on Marvel’s chief creative engine, president Kevin Feige, is starting to catch up with him, according to the authors of the recently published MCU: The Reign of Marvel Studios.

Feige was the man most responsible for delivering Marvel’s string of blockbusters, starting with 2008’s Iron Man, and turning the studio into a dominant player in Hollywood. But as Marvel’s ambitions grew and the studio churned out an increasing number of streaming shows to go along with its usual theatrical releases, it has struggled to stretch Feige’s talents, Dave Gonzales, Gavin Edwards, and longtime pop culture journalist and chronicler of superhero movies Joanna Robinson write in MCU. Since Disney—Marvel’s parent company—launched its Disney+ streaming service in 2019, Feige has been overextended, they argue.   

Under Feige’s leadership, Marvel has produced 33 movies, nine shows, and grossed around $29 billion in total box office sales in 15 years. Known for characters’ trademark quippiness, over-the-top visual special effects, and inspiring legions of nerds into thinking they’re cool, Marvel movies have meant big business because they guaranteed a minimum level of quality that promised box office receipts ranging from good to historic. Marvel’s Avengers: Endgame became the highest-grossing movie ever at its 2019 release when it made $2.8 billion worldwide. Even the studio’s lowest-grossing movie ever, 2008’s The Incredible Hulk, was still plenty profitable, making $265 million on a $150 million budget. 

Friday’s release of The Marvels shows that the weight placed on Feige’s shoulders is starting to exact its price—from Marvel’s box office returns.   

‘The irreplaceable man’

After Disney bought Marvel for $4 billion in 2009, Feige earned Bob Iger’s trust almost instantly. The longtime Disney CEO recognized the value Feige brought to Marvel and was more than happy to keep him in place once the acquisition was complete, according to the book. In one of their first meetings, Feige laid out his vision for the perpetually sequelized movies that would culminate in regular crossovers, and Iger replied: “seemed brilliant to me.”

Feige is widely credited with being the creative steward behind Marvel’s plethora of movies and shows. Having a single individual responsible for the studio’s entire output was critical to executing Marvel’s revolutionary idea of making all of its movies and television shows interconnected. (Although the idea to make endless sequels was the brainchild of David Maisel, who MCU calls a critical and unfairly overlooked executive in Marvel’s history.) Marvel needed at least one person who could see the entire chessboard. 

Feige is even credited with conceiving of the “Marvel method,” a new way to produce movies in which reshoots were essentially baked into the development process, Robinson says. After a first round of filming, Feige would watch a cut of the movie, offer feedback, and then send the actors and crew back out to the set for another round of shoots. 

Until Feige was forced to increase Marvel’s output to precarious levels, the process had always worked because he has “such an unerring compass for a blockbuster movie tastemaking,” Robinson says. 

‘Stretched too thin’

But when Disney released its Disney+ streaming service in 2019, the studio was forced to increase its output to satisfy the streamer’s ever-growing demand for content. 

“In the era of Peak TV, Disney+ needed Marvel shows fast, and to stay competitive, it wanted a lot of them,” the book’s authors write. “Feige essentially had a blank check for television programming, so long as he could deliver volume.” 

Having to crank out a raft of streaming shows, in addition to Marvel’s regular slate of movies, left Feige’s creative guidance in short supply. 

“Kevin [Feige] can’t physically do what he used to do for [just] two to three movies a year, for two or three movies a year plus all these Disney+ shows,” Robinson, who also cohosts the superhero-themed House of R podcast, tells Fortune. “You see this massive dropoff in quality across the board, because the man is stretched too thin.”

Marvel and Disney, anticipating the new demand for content might affect quality, tried to head off the problem by establishing the Marvel Parliament, a committee of longtime producers that was meant to take over some of Feige’s duties, according to the book. But even this new consortium struggled to replicate Feige’s talents. 

“They’re brilliant people, they have great ideas, but they can’t do what Kevin can do,” Robinson says. 

The aura of creative invincibility that surrounded Marvel has been punctured of late. Some recent Disney+ shows, including Secret Invasion and She-Hulk: Attorney at Law, have been critically lambasted—words like “dull,” “uninspired,” and even “soulless” were thrown around. Disney CEO Iger himself seemingly acknowledged that the quality of some of Disney’s latest releases was lackluster, telling investors earlier this week it “wasn’t really up to the standards that we set for ourselves.” He made a similar point in July. 

“They’ve got a successor problem,” says Edwards. “There is no obvious person I think is going to take the next step. [Feige] is the irreplaceable man right now.”

That’s partly because of Feige’s unique mix of entertainment business acumen and creative disposition, according to Robinson and Gonzales. Before becoming an executive, Feige was an aspiring director who was rejected from film school five times. 

“We don’t know how you teach taste to someone,” Robinson says. “It’s too artistic, what Kevin Feige does, and that is extremely unusual for an executive or a head of a studio. I think he could teach someone to, like, run a meeting or manage relationships or all these other things that an executive has to do, but I don’t think he can teach the art of what he does.”