In 2017, Viacom turned to one of Hollywood’s most seasoned and respected executives, James N. Gianopulos, to revive its flatlining Paramount Pictures operation. Mr. Gianopulos quickly stabilized the 1910s-era studio — repairing relationships with filmmakers and producers, building a thriving television division from near-scratch, and restoring Paramount to profitability.He was ousted on Monday, with his status as the consummate Hollywood insider having curdled into a liability, at least to ViacomCBS, the conglomerate that owns Paramount, where streaming, streaming, streaming is the new currency of the realm.Brian Robbins, 58, who runs Viacom’s children’s television business, will succeed Mr. Gianopulos, 69, as chief executive of Paramount Pictures, ViacomCBS said. Emma Watts, 51, the president of Paramount’s Motion Picture Group, was notably passed over for the job.The reversal of fortune for Mr. Gianopulos, who had two years left on his contract, did not shock the movie capital, where speculation about his standing inside ViacomCBS had been gossiped about for months. Shari Redstone, who controls the company, had signaled in private that Mr. Gianopulos had become a frustration. In particular, he had, at times, resisted a ViacomCBS effort to prioritize the Paramount+ streaming service at the expense of ticket sales and theaters. Big-screen releases remain of crucial importance to studio partners like Tom Cruise, who stars in Paramount’s “Mission: Impossible” series and coming “Top Gun” sequel.Shari Redstone, the chair of ViacomCBS, in July. She has been pushing the company to prioritize streaming.Kevin Dietsch/Getty ImagesBut the ouster rattled the film business nonetheless. It was seen by some as barbarous — tradition holds that senior statesmen get to write their own endings. And it added to a changing of the guard: Ron Meyer, a longtime film power at Universal, left last year amid a sex scandal, and Alan F. Horn, the chief creative officer at Walt Disney Studios, is widely expected to retire in the coming months.Bob Bakish, the chief executive of ViacomCBS, said in a statement that the leadership change would “build on Paramount’s strong momentum, ensuring it continues to engage audiences at scale while embracing viewers’ evolving tastes and habits.” He said Mr. Robbins was an “expert” at developing franchises by “leaning into the unique strengths of new and established platforms.”Mr. Bakish called Mr. Gianopulos “a towering figure in Hollywood” and thanked him for revitalizing Paramount. In the same statement, Mr. Gianopulos recounted a list of major changes he had successfully navigated over his nearly 40-year career — such as the introduction of VCRs and online film rentals — and wished Mr. Robbins “all the very best success.”For many film industry stalwarts, Mr. Robbins is an affront to their identities; he comes from television, said while holding one’s nose. Mr. Robbins has experience as a movie producer and director. But much of the Hollywood establishment also looks down on that part of his résumé, which includes “Norbit,” a commercially successful but critically reviled Eddie Murphy vehicle from 2007. Not exactly Oscar bait.Mr. Robbins gained fame as a young actor in the 1980s by playing a mulleted rebel on the ABC sitcom “Head of the Class.” In the 1990s and 2000s he worked as a television producer (“Kenan & Kel” on Nickelodeon, “Smallville” on the WB) and a film director (“Norbit,” “Varsity Blues”).By 2009, however, Mr. Robbins started to become disillusioned with Hollywood. Younger audiences — his specialty — were living online. He began experimenting with low-budget films starring YouTube personalities like Lucas Cruikshank (a.k.a. Fred Figgelhorn) and started a YouTube channel, AwesomenessTV, aimed at teenage girls. In 2013, Jeffrey Katzenberg, who was then running DreamWorks Animation, bought AwesomenessTV for about $33 million. (Mr. Katzenberg remains a mentor.)“There is no movie business anymore!” Mr. Robbins was quoted by Fast Company as saying in 2013. “The model’s broken, and I see that as an opportunity.”Mr. Robbins was named president of Nickelodeon, which is also owned by ViacomCBS, in 2018. He has become known inside Viacom as a plain-spoken, never-say-die futurist who believes that Paramount+, the company’s relatively small streaming service, must be supercharged. Mr. Robbins has eagerly rerouted new children’s programming toward Paramount+ and away from Nickelodeon’s traditional cable channels. One such show, a reboot of “iCarly,” has been a hit for the streaming service.Mr. Gianopulos, or “Jim G” as everyone in Hollywood refers to him, will remain a consultant until the end of the year, ViacomCBS said. “Jim is nothing less than legendary in this business, and I am humbled and grateful to him for his years of mentorship and friendship,” Mr. Robbins said in a statement.Mr. Robbins will continue to lead Nickelodeon, ViacomCBS said. But he will not get all of Mr. Gianopulos’s portfolio; Paramount Television Studios will now report to David Nevins, the chairman and chief executive of Showtime Networks. More