There were zillions of sequels from the earliest days of movies. The form of them gradually evolved, though.
The oldest sequels took a hit, threw the same stars in the same plot, but slightly tweaked the name - or not even that. Pearl White, the serial heroine, starred in The Perils of Pauline in 1914. Later that year they redid the story as The Exploits of Elaine (1914-1915) and followed that in 1915 with The New Exploits of Elaine and The Romance of Elaine. You can’t think of these as anything but sequels.
In the sound era, Universal, then a relatively minor studio, made its name by pumping out horror movies in the 30s. These included Frankenstein, Bride of Frankenstein and Son of Frankenstein. Similarly, they made *Dracula *and Dracula’s Daughter. They rebooted all the series in the 40s and made every monster the star of a series of sequels.
Even closer to modern-day sequels were the six Thin Man movies, all starring William Powell and Myrna Loy, and the seven Road movies, all starring Bob Hope, Bing Crosby, and Dorothy Lamour.
B-movies series by the dozens existed. Detective/crime fighters like The Saint, the Falcon, Boston Blackie, Charlie Chan, Ellery Queen, Philo Vance, and a page full of others each had a half dozen of more movies made about them. So did western stars Hopalong Cassidy and Roy Rogers and more.
In some ways, we’re beginning to blur the distinction between sequels and series. Modern sequels often - but not always - make specific reference to the earlier movies. Oldtime series seldom - but sometimes - did.
“More of the same” has been Hollywood’s motto since day one. There have always been sequels. Always. There have always been franchises. Mickey Rooney and Judy Garland were a franchise. Abbott and Costello were a franchise. MGM musicals were a franchise. We’ve just skewed the definition a bit to fit our times, but the underlying structure of the planning, promotion, and production of sequels and franchises really hasn’t budged a bit since Edison made them.