Cash Cow Franchise: The three movies were huge financial successes. Even the third, which was less well-received by critics or fans than the previous films, was still a success, to the point of being the highest grossing domestic release of 2007, and the highest grossing Spider-Man film worldwide to date.
Executive Meddling: This is what ultimately killed this iteration of the film franchise. Spider-Man 4 was green lit with Raimi wanting to do a storyline with Peter and Mary Jane's wedding and the Vulture as the main villain as well as Dr. Connors becoming The Lizard. However, Sony insisted on doing a story and script that they came up with. According to reports, it had Peter married to Mary Jane for several years, the Vulture being killed by Spider-Man, inciting the wrath of Felicia Hardy — who would here not be the Black Cat, but the Vulture's daughter called the Vulturess, with whom Peter would essentially cheat on Mary Jane with; and most notably, the ending would have had Mary Jane leaving with her and Peter's baby, and "Spider-Man No More" happening again. Raimi absolutely hated this idea and refused to do it which led to him being fired. Maguire and Dunst told the studio they would not reprise their roles unless Raimi was brought back, so the studio decided to cancel the film and do a Continuity Reboot with The Amazing Spider-Man.
For Tobey Maguire and Kirsten Dunst. It was also James Franco's first big role, but he never became a main event player until later on. Some actors and actresses in the cast also got additional boost because of the trilogy.
It was also something of the same for Willem Dafoe who until then had made a name for himself in The '80s (in Platoon and The Last Temptation of Christ) but appeared after that largely in independent films and this was the first film in which he appeared that was a major blockbuster (he had previously appeared in the forgettable Speed sequel that didn't do as well) and it led to more prominent parts later on.
There had been numerous attempts at making the trilogy a part of a Shared Universe of some kind in the past. The first attempt occurred during production of the original movie, as it was planned for Hugh Jackman to make a cameo as Wolverine in the film, but said attempt fell apart at the last minute due to the production crew being unable to locate his X-Men costume. Later on, Sony considered making a crossover film between Spider-Man and Ghost Rider, but decided against it due to the performance of the latter film. Finally, sometime around 2007, there had been two attempts that, had they succeeded, would've made the trilogy a part of the Marvel Cinematic Universe. One of the drafts for Iron Man mentioned that Tony Stark had some degree of involvement in the creation of Otto Octavius' robotic tentacles, and Tobey Maguire was planned to cameo as Peter Parker in The Incredible Hulk. Due to Marvel Studios and Sony being unable to come to an agreement, both attempts fell through.
The script for the fourth film which caused Raimi to leave. According to reports, it had Peter married to Mary Jane for several years, Adrian Toomes replacing Jameson as head of the Daily Bugle, Spider-Man deliberately killing the Vulture, inciting the wrath of Felicia Hardy — who would here not be the Black Cat, but the Vulture's daughter with the absurd name of "The Vulturess", with whom Peter would essentially cheat on Mary Jane with; and most notably, the ending would have had Mary Jane leaving with her and Peter's baby, and "Spider-Man No More" happening again. According to sources, Raimi was repeatedly vocal about how much he hated this script.
Years ago, James Cameron was going to do the Spider-Man movie with Arnold Schwarzenegger as Dr. Octopus. During this time, there was also another R-rated treatment that had a villain who was a Composite Character of Norman Osborn and Electro, would feature Spider-Man killing criminals, and would have MJ and Spidey having sex on a big web while Peter kept his mask on.
When Cannon Films attempted to develop the film in the mid-'80s, they hired Leslie Stevens to write the script, who had very little understanding of who Spider-Man was. As a result, the draft he had turned in was a film that would've been the ultimate In-Name-Only adaptation, as it was written as a Body Horror movie that would've depicted Peter Parker as an ID badge photographer that gets exposed to a heavy amount of radiation by a Mad Scientist and is transformed into a grotesque eight-armed monster. The mad scientist would've attempted to make Peter a part of an army of mutant monsters that he had created, but Peter refuses and battles the mutant army and, at the end of the film, he and the scientist would've been killed in an explosion. As a cherry on top, this film would've been directed by Tobe Hooper (of Texas Chain Saw Massacre and Poltergeist fame). Stan Lee, appalled by the script's treatment of his creation, demanded a rewrite.