Sequels
There was a time when they were considered a cheap, exploitative means of cashing in on an original and popular movie.
Sure, there were exceptions. The Godfather movies, the French Connections, even the Dirty Harry films were top quality. And during earlier years we had movie serials like Tarzan, Sherlock Holmes, Charlie Chan and the like. But for the most part, especially in the 80's and during the horror boom of sequelitis, they were...nasty.
But Star Wars bucked the trend.
As far as sci-fi epics go at least. How come a film like Empire could come with such amazing expectation? Its budget was twice that of its predecessor, its scope broader and more ambitious. But the studio, licensors and attached parties didn't see it. We all know there's half the merchandise for ESB than there is for Star Wars. No one thought lightning could strike twice.
But they were wrong.
Is it because those films weren't numbered? Star Wars 2, 3, 4 et al? Most franchises in their day numbered their films. Police Academy 6, Halloween 4, Nightmare on Elm Street 5, Star Trek 3 etc, etc, etc. Star Wars films had not only a huge fan base but quality - and original sounding titles.
But there must be more than that. Among the many moulds that Star Wars broke argueably the biggest one was making sequels as legitimate as their forefathers. Temple of Doom is as big a film as Raiders. Superman 2 surpasses its progenitor. And once we move forward a couple of decades to the prequels...
Do we even think of Revenge of the Sith as the 6th Star Wars film?
Any other franchise gets to six, usually we're scraping the barrel but with Star Wars it was the most important episode of the entire saga. And other franchises have followed suit. Every consequent episode in the excellent Harry Potter series has had broader scope, bigger budgets and great returns. Same for Spiderman.
Some got it wrong. The Matrix sequels were a disappointment, but made amazing money. The Lord of the Rings series were an ambitious project which worked amazingly well.
But without the proven template of Star Wars, would studios even attempt to embark on such ambitious ideas?
In short, as well as being at the birth of the summer blockbuster (May 25th, Star Wars day in Hollywood, is the official start of the summer blockbuster season), kicking off the licensing boom, bringing orchestral music back to the forefront of modern cinema, exploding the SPFX era and launcing many great and influential careers, did Star Wars, or more specifically the Empire Strikes Back, solidify the art of making great sequels.
And making them desirable for studios to invest in.