The illusion of change works best if the readers don’t know how the story is going to end, or where the characters are going to end up. One of the things that worked about One More Day is that it muddied the water about whether Peter and MJ will end up together, and other factors as well. That provides a lot of flexibility in the future, in both the regular Spider-Man comics, and other renditions of the story.
For example, the first issue of the post-Ultimatum Ultimate Spider-Man had Peter Parker fooling around with Gwen Stacy. This did upset some readers who liked a book where Peter and MJ were a couple, especially after the events of One More Day. But it represented an advantage of getting rid of the spider-marriage in the 616 universe: the ending of the story (or this facet of it) was no longer certain. It wasn’t entirely clear whether Peter and MJ would rekindle their relationship in the Ultimate comics.
This means you don’t know how the new Spider-man movies are going to end. If in an upcoming film, an actress playing the Black Cat has incredible chemistry with whoever’s playing Peter Parker, the Director will be better able to take advantage of that, now that no one will be able to argue that the “real” (and by this I mean, the 616 Spider-Man whose adventures have been published since Amazing Fantasy #15) is happily married in the latest Amazing Spider-Man issue, so it wouldn’t be clear that the movie Peter Parker will get back together with Movie MJ eventually.
When the first Spider-Man film ended, most of us knew Peter and Mary Jane would eventually be a couple because they were married in the source material. This hindered the ability to have any significant “Will they or won’t they?” tension, and took away from the ending of the movie.
Many readers still think that the story is guaranteed to end with Peter and Mary Jane together, but it’s something that is increasingly ambiguious. While getting rid of the sense of inevitability may take more time, it’s pretty much impossible if the “real” (or original or classic or whatever term you want to use) Spider-Man is happily married. The more distance we have from OMD, the less inevitable a reversion will seem.
The muddied waters also help in the Amazing Spider-Man comic book. When a plot point is resolved, there’s an expectation that the characters will get to the next step. When Peter graduated high school, the understanding was that he would soon go to college. When Peter and MJ were married, there was an expectation that at some point they would have a family, and there were reasons for Marvel not to go with that approach.
Prior to One More Day, the writers still had the freedom to explore other aspects of the character’s life, including different career paths and education choices. You could argue that since the comics featured Peter graduating High School and College, it should be okay to show him taking other fairly normal steps, including marriage. But there are a few distinctions. Graduation was something that unambiguously opened up new storytelling opportunities. For a guy with his intelligence and background, graduating college was even more likely than getting married. But the problem wasn’t just that the character got married, but who he got married to. If Marc Webb wants to make Peter Parker a student of NYU rather than the fictional Empire State University, most readers wouldn’t care. But they would have been bothered if a future Spider-Man film ends with the character marrying someone else. Or they would believe that this new marriage was still doomed to failure, because everyone knew that Peter was going to end up with MJ at some point.
One More Day muddied the water on two other developments which could have been definitive moments for the series. Harry Osborn’s resurrection meant that his story wasn’t guaranteed to end with a last-minute redemption, when the character died to save his best friend. The remasking meant that even the public’s knowledge of Spidey’s identity was a genie that could be put back in the bottle. The ability to shut some of the monsters back into Pandora’s box is crucial to the illusion of change.