LOS ANGELES — Every three decades, or roughly once a generation, Hollywood experiences a seismic shift. The transition from silent films to talkies in the 1920s. The rise of broadcast television in the 1950s. The raucous “I Want My MTV” cable boom of the 1980s.

It is happening again. The long-promised streaming revolution — the next great leap in how the world gets its entertainment — is finally here.

Streaming services, of course, have been challenging the Hollywood status quo for years. Netflix began streaming movies and television shows in 2007 and has grown into a giant, spending $12 billion on programming this year to entertain more than 158 million subscribers worldwide. There are 271 online video services available in the United States, according to the research firm Parks Associates, one for seemingly every predilection — Pongalo for telenovelas, AeroCinema for aviation documentaries, Shudder for horror movies, Horse Lifestyle for equine-themed content. (Offerings include a series called “Marvin the Tap Dancing Horse.”)

While all this was happening, however, the three biggest old-line media companies — Disney, NBCUniversal and WarnerMedia — largely stayed on the sidelines. Charging into the streaming fray would mean putting billions of dollars in profit from existing cable networks like USA, Disney Channel and TBS at risk. Building video platforms of the size needed to compete with Netflix and Amazon would be frightfully expensive. And mastering the underlying technology would require a sharp learning curve. Better to bide their time. When it became clear that protecting their existing business model was more perilous than embracing the future, no matter now disruptive in the near term, they would act.