It is practically a cliche to complain that modern movies and television are creatively bankrupt. I won't argue the point. But one piece of evidence often used to support that notion is the reliance of Hollywood and TV on spinoffs — shows or movies built around characters introduced in another show or movie. Trouble is, spinoffs are not a new development, and were just as common during times of great creativity.
First, a clarification. There's a sneaky method used in the TV industry, in which a character is introduced into an existing show (typically a hit series) solely in order to serve as an "embedded pilot" for a show about that character. One example famous among Trekkies is the old Star Trek episode "Assignment: Earth," which centered on the mysterious time and space traveler "Gary Seven," along with his goofy Sixties secretary and his cat (who might have been a shapeshifting alien). The only reason Gary & co. appeared on Star Trek at all was to piggyback on the success of Kirk & co.
Another example is the 1970s sitcom All in the Family, which famously spawned several spinoffs, including The Jeffersons and Maude. But while Sherman Hemsley as George Jefferson was a semi-regular character on All in the Family before getting his own show, Bea Arthur as Maude Finley appeared in exactly two episodes. I suspect those were more of an example of embedded pilots than an actual spinoff.
Once you start looking, it turns out that spinoffs predate television. The "Ma and Pa Kettle" series of movie comedies, which ran from 1947 to 1957, were spinoffs. The Kettles originally appeared as supporting characters in the Fred MacMurray-Claudette Colbert comedy The Egg and I, and were so popular they got their own movies.
The same thing happened with the "Dead End Kids" (a.k.a. the Bowery Boys, a.k.a. the East Side Kids, a.k.a. the Little Tough Guys) — Leo Gorcey, Huntz Hall, and half a dozen other youthful actors. They started out as fairly serious supporting characters in the 1935 Broadway play Dead End. The actors playing the Kids reprised their roles in the 1937 movie of the same name . . . and then went on to play the same characters in more than sixty movies over the next twenty-odd years, until the "Kids" were eligible to join the AARP.
Across the Atlantic, the 1938 film The Lady Vanishes featured two comic-but-adventurous cricket enthusiasts called Charters and Caldicott (played by Naunton Wayne and Basil Radford). They were so popular that they were brought back for the movie Night Train to Munich* and ten more films over the next decade, plus a radio series.
But to find the original spinoff we have to go much farther back.
In or around 1597, a chap named William Shakespeare (you may have heard of him) wrote Henry IV, Part 1. To relieve the somewhat numbing historical drama of the play he created his best-known comic character, the dissolute but charming Sir John Falstaff. If you're not familiar with the character, read the play.
Falstaff returned in Henry IV, Part 2 (along with just about all the other characters), and proved so popular that Shakespeare decided to spin him off into his own play, The Merry Wives of Windsor. A charming legend (not recorded until a century later) claims that Elizabeth I herself insisted that Shakespeare write another play about Falstaff — there being no network or studio executives available in the Sixteenth Century.
Personally, I suspect there were probably spinoffs well before Shakespeare wrote Merry Wives of Windsor. In any serial medium, creators will find a way to squeeze some more audience interest out of any popular character. The results may be good or bad, but don't say much one way or the other about the creative bankruptcy of the medium.
*An absolutely mind-boggling movie. It's got Rex Harrison as the most hilariously mis-cast super-spy in cinema, Paul Henreid as the evil doppelganger of his Casablanca character Victor Laszlo, and poor Charters and Caldicott left to do the actual heroics in the film.