Multiverse theory says that there’s an untold quantity of universes out there, and each one might be different based on a single choice that one person made. There’s a universe out there where, for example, I live in outer space, because the Library of Alexandria never burned down and the advancement of human knowledge wasn’t set back by centuries. If you had the opportunity to fix this, would you? That’s a pretty huge reset, so it’s hard to begin to imagine the ramifications of doing so. What about, say, going back three hours to create a timeline in which your friend’s pet isn’t murdered and your godfather doesn’t receive the Dementor’s Kiss? This presumably creates a better future, as it results in Sirius living, which gives Harry access to Grimmauld Place, which gives him access to Kreacher and the story of Regulus, etc. As an author, resetting the timeline is something you should only do in the worst possible circumstances, and even then, it’s hard to argue that retconning a given set of circumstances that are bad for your characters is worth retconning an entire universe for.
A good example of a story where this works is Jumanji. The story clearly shows us that Alan Parrish’s disappearance from the real world into the game of Jumanji had a damaging effect on the whole town. His father, the owner of the shoe factory that was the base of the town’s economy, went bankrupt pouring money into his search for his son. The factory worker whose inventive idea was going to revolutionize the shoe company ended up getting fired, and had to find work as a police officer. The factory closed, and when you see the town twenty years later it’s ramshackle and many of the people are homeless.
Sarah, the girl with whom Alan played the game, has spent years suffering from PTSD and social anxiety after being shunned by the town for claiming her friend was sucked into a board game. Judy and Peter, the children who find the game and play it to bring Alan back out, are orphans, their parents having died years earlier while traveling. The game itself has horrifying consequences, destroying Alan’s house and most of the town, and transforming or hurting its players in terrible ways.
By finishing the game and resetting the timeline back to when he was a kid, Alan not only creates a better future for himself, but for the whole town as well. Alan and Sarah, aware of what happened in the game even after resetting the old timeline, are able to manipulate the new one so that all the negative effects of the game are negated. Alan fosters a better relationship with his father, whose shoemaking business booms. Alan and Sarah get married and hire Judy and Peter’s parents, ensuring that they don’t go on the vacation in which, in the other timeline, they died. It’s patently obvious that it was the best option all around to reset the timeline. From a writing point of view, retconning the events of the game while leaving Alan and Sarah with their memories allows them to grow as people outside of the game. It turns the erasure of a timeline into an important character moment.
An example of a story where this is more questionable is Homestuck. Within the Homestuck story, there is the alpha timeline and then there are doomed timelines. Homestuck takes place in a semi-sentient video game session. Whenever the game senses that conditions are no longer amenable to winning, such as when a crucial player has died or a necessary quest fails, that progression of events becomes doomed. If, however, a player can identify the event which caused the doom-ing and rectify it through time travel or some other kind of meddling, the timeline continues as planned and the players keep on going. This doesn’t undo the doomed timeline, unfortunately; it simply assigns the role of ‘alpha’ timeline to a universe where no one fucked up.
For example, in the original timeline, John, our protag, takes some bad advice from one of the trolls and dies. His friends play on, but without his powers and leadership, they’re unable to escalate the in-game conflict in the way they were required to do. After much existential contemplation, Dave, who has time travel abilities, goes back in time to warn John not to take Terezi’s advice. In doing so, he condemns the timeline from which he came to doomed-ness; it’s no longer the alpha, and his remaining friends in that timeline will continue on without the consolation of knowing it will mean anything in the long run. Meanwhile, that Dave will continue to exist alongside the new alpha timeline’s Dave, and struggle with all the psychological effects that come with playing second fiddle to a less advanced version of himself. This is our first real glimpse at the way alpha/doomed timelines work in Homestuck, but it isn’t the last.
Thousands of pages later, with nearly half the cast dead or dying and no forseeable way out, John, Terezi, and Roxy again reset the timeline in a move that retcons several thousand pages of the story. By going back several acts and assuring that one person survives, they create a new alpha timeline where the odds are tremendously more in their favor. While the narration leads us to believe that retconning a huge portion of the timeline was the right and only way to salvage the adventure, the fact that the story is still unfinished leaves fans uncertain if it’ll all work out. In fact, many fans took umbrage at a move that they felt, in essence, made the last several years of their reading experience pointless. Did author Andrew Hussie just write himself into a hole and decided that a convoluted retcon was the only way to save his story? (The fact that the character he revived is well-known to be his favorite makes this even more suspicious.) However, we do see that, post-retcon, everyone does seem to be happier and less dead, and the opportunity for the story to resolve in a positive way is there in a way that it previously was not.
In the end, the ability to reset a timeline is a power that ought not to be used lightly. No matter how bad your characters’ current situation looks, it’s hard to say for sure that it’s ever the right decision, because you have to be able to argue convincingly that the reset world will be better off than the one they started out in. Because the stakes as well as the chances for serious complications are so high, it’s wise to err on the side of ‘this is the best of all possible worlds’ unless you’re really, really sure it isn’t.