These days, every cool science fiction universe is a "franchise," meaning that it's a piece of intellectual property separate from any particular creator or even set of characters. It wasn't always that way — everything changed in the 1980s.
Long before the 1980s, there were cash cows that got milked to death. Hollywood has been cranking out endless sequels and remakes, for almost as long as there's been celluloid. There were slews of Frankenstein and Dracula movies prior to the 1980s. The James Bond series started playing musical chairs with its stars and directors in the late 1960s. Batman and Superman were played by a few actors. The 1970s saw TV series of Logan's Run and Planet of the Apes, with new actors playing characters from the movies.
What was different in the 1980s? A few things. This was the first time studios really started to exploit the ability of popular entertainments to leap between different platforms — toys to cartoons to video games, etc. And this was the first time studios really started to grasp that you could have Star Trek without Captain Kirk. It saw the rise of the huge comics company crossovers, like Crisis on Infinite Earths and Secret Wars. And big entertainment companies started to merge into super-conglomerates.
Universes could exist without their most famous characters
You might have had different actors playing James Bond or Batman prior to 1980, but nobody ever thought of doing a James Bond movie without James Bond. The rise of Star Trek: The Next Generation, in particular, proved that a popular universe doesn't need its most well-known characters to succeed. A whole generation grew up thinking of Jean-Luc Picard as the captain of the Enterprise. Star Trek went from a near-death experience in 1980 to becoming the ultimate example of a successful franchise by 1989, with a hit TV series and movie series featuring two different casts.
To a lesser extent, the 1980s also saw a Ghostbusters series which didn't feature Venkman or any of the other characters from the movie (although it was actually a spin-off of a 1975 series called The Ghost Busters.) Galactica 1980 managed to continue Battlestar Galactica without Starbuck or Apollo (with limited success), and Beyond Westworld also introduced a new cast of characters.
But really, Star Trek is the first great example of a science fiction franchise that transcends any particular character or concept.
This was also the era when sequels were great
List the best science fiction movie sequels of all time, and you'll inevitably wind up with a ton of films from the 1980s, including Star Trek II, Empire Strikes Back, Back to the Future 2, Mad Max: The Road Warrior and Aliens. (Although the decade also saw Star Trek V and Ghostbusters 2.) You could easily argue that the 1980s was the era when the movie sequel came closest to being an art-form, instead of something like Futureworld. Why does this matter? I freely admit I'm spitballing a bit, but it seems like the existence of a ton of actually good sequels might have raised the expectation that movie series could go on and on, and on. And on.
In the recently released Making of The Empire Strikes Back book, there's a lot of stuff about the fact that many studio execs didn't really expect a sequel to Star Wars — one of the most successful films of all time — because such sequels were considered unusual at the time. Reading that stuff, it feels like you're learning about a foreign culture. Nobody could imagine a megahit on the scale of Star Wars not having a sequel, in this day and age.
Toys became TV shows, games became comics
Sure, there were tons of TV shows and movies based on comic books prior to 1980 — but I don't think there were cartoons based on toys. The 1980s saw the invention of a whole new type of animated series — one which took successful toys like Transformers, He-Man and G.I. Joe as the jumping-off point for a whole detailed universe full of locations that could be turned into new playsets. Meanwhile, the success of the Dungeons & Dragons role-playing game led to a cartoon series. And then there were the comics based on video games, including the surprisingly great Atari Force.
The idea that a cool concept could translate to an infinite number of platforms took off like never before in the 1980s, and paved the way for our current era, where a Stretch Armstrong movie is in the pipeline and the Battleship film is being made already.
Comics gave us mega-crossovers and the invention of the "reboot"
Comic-book universes had always sucked in readers with a high degree of crossovers and tight continuity — the same villains turned up in The Fantastic Four and The Avengers, and Batman and Superman hung out all the time. The 1980s turned this up another few clicks, with the invention of the mega-crossover like Crisis on Infinite Earths, Millennium, Legends, Secret Wars, Secret Wars II, Inferno, Atlantis Attacks and Acts of Vengeance. These lasted way longer than an old-school crossover, and required you to buy tons of different titles and keep track of tons of subplots to make sense of them.
Even more important to the development of today's mega-franchises was the invention of the "reboot," which arguably started after DC's Crisis on Infinite Earths, when DC restarted its major titles with a new, streamlined backstory. You can't have a franchise that lasts for decades, with the ability to swap in new characters and get rid of worn-out ideas, without the "reboot." By proving that you could publish a Superman comic in which he was never Superboy, and a Batman comic where he never caught his parents' killer, DC showed once and for all that backstories were mutable. Casual readers/viewers would barely notice that you'd changed things up, and fans would be fascinated by the chance to learn a new version of the same old story.
Without the ability to "reboot" continuity and make wholesale changes, a franchise will tend to run out of steam over time — but now, comics companies revamp their origin stories on a regular basis, while Spider-Man's getting a movie reboot less than a decade after his film series began. Thank the 1980s for launching this trend.
Finally — and this might be the most important factor of them all — the 1980s saw an unprecedented mergers-and-acquisitions boom, funded in many cases by debt or by the assets of the target company. And this was the beginning of the true entertainment mega-conglomerate, with stakes in television, movies and other platforms.
Time Inc. and Warner Bros. merged in 1989 to form Time-Warner, while News Corp. bought out 20th Century Fox in the mid-1980s, and set about creating a Fox television network. And it stands to reason that the bigger and more diversified a corporate entity is, the more interest it'll have in properties that are infinitely monetizeable.
TNG publicity photos via Trekcore.