My position is that the current status against retconning is too firm and needs to be softened.
Before we talk about retconning, it's important to examine continuity and the various ways that storytellers have tackled continuity. The subject is huge and worth its own well researched book. I'm not so thorough, so I'll hand wave without sources and expect you to believe me.
When telling stories to kids, retconning is part and parcel of storytelling. Either the teller or the listener suggest some alteration in the middle, so the entire story changes with that. Often the change is abrupt, such as suddenly inserting a servant where no servant existed before, and other times its done with more finesse, so that the addition simply flows in, but they're all retcons equally. Across stories, facts are also changeable, but only with agreement. "They should live in an igloo," the kid might say. They didn't live in one before, but now, hey, they've always lived in an igloo.
Loose continuity is familiar to most adults. Through the mid-1900s, comic books and TV shows were built on loose continuity, where the characters were familiar but each story was self-contained. Sometimes those shows even replaced actors, such as Bewitched, where Darren changed between one season and the next. Such a change was unusual, but not outside of bounds. Plays changed actors all the time, even on the radio, so that generation didn't blink when TV actors changed. The characters that they played remained the same.
What's important in most stories, even those with loose continuity, is that the continuity of character takes precedence over everything else. It's not that the character has a deep history, it's that the character has certain well defined traits that make that character. King Arthur is noble, good, wields Excalibur, and holds court at the Round Table in Camelot. Batman wears a mask, is a super-ninja, and relies on his gadgets. Darth Vader is bad, smart, uses the force, and kills his own people when angry. Around those traits, you can build any number of stories not necessarily connected.
Serials hard larger, longer continuities, but even they had loopholes for retconning. In a serial, you have a story that develops over a longer period of time, one that the writers haven't necessarily finished plotting, so they have to be clever or devious or sometimes brazen when changing the internal continuity of the series. There are ways to do it. "Little did our hero know that ..." is a classic example. There was some person or situation that didn't appear in the primary story, gets inserted later, and is then used to (presumably) good effect in the story. However, if there are multiple serials, one serial does not necessarily override another. The second serial may retcon the Dickens out of the first serial. (That's a serial joke. Dickens was a king of serials.) In fact, the audience expected retcons out of follow-on serials because everyone did that.
Audience expectation is a big deal when it comes to continuity. Before modernity, audiences expected some degree of retconning, if not a story flat out disregarding what came before. As long as the story at hand held together, the bigger issue of overarching continuity didn't matter.
I don't know what started the modern notion of absolute continuity. I don't really know what cultural zeitgeist caused the devolution of retconning from a regularly used tool to a disreputable tool. Whatever it was, it affected the writers just as much as the readers, because the writers bought into this stiffer continuity. Certainly the creation of larger story structures played a part. The desire fans to know what was going on and how everything fit together, and the stories between, drove this. The thing is, I'm not sure that the fans themselves knew that their desire to know more stories and exactly how they fit together would produce increasingly rigid story structures, and once fans got used to that, the looser structure of previous decades seemed slipshod.
So now we're at a time when retcons seem like a betrayal to fans. To retcon feels like some part of the story has been taken from you, and people absolutely hate the feeling that something's been taken away. Retconning now produces great emotional reaction and gnashing of teeth. To retcon is to break an internal rules set, to cheat, to betray, to ruin. It is to be shunned at all cost.
Yet, we still retcon. Its use has gone from implicit in any set of stories to explicit. People are forward about it. Moves and comics now "reboot" a universe. For now, this is working, but it doesn't work for everything, and it certainly doesn't work for the past.
Some retcons happen because the world around us changes. Anne McCaffrey began her Pern series at a time when women's lib was just beginning. She had her male dominated world with a woman pushing to free herself from restraints. As she build a female fan base, they asked, "Why can't women ride dragons?" The only answer was that a book written years before said that they couldn't. I don't know about you, but that's a pretty bad answer. Binding a writer to a poor or outdated decision because retcons are bad is the single worse excuse for keeping continuity that I can think of. Writers aren't perfect. They make bad decisions. That is exactly the case where retcons should be used. But like the writers of her day, she attempted to use implicit retcons, one that made sense inside the story. To this day, there are people mad at that retconning, and plot weirdnesses that haven't quite gone away.
Another example was Earthsea, where LeGuin struggled for years against the poor decision that she made that said that only men could be wizards. No close reading of her Earthsea books is required to see that she regretted that decision.
The thing is, both these series could have easily fixed their problem and given us engaging stories if only retconning had been more tolerated. In these cases, the avoidance of retconning caused more woe than it solved. But who knew? Nobody could guess the future.
Even worse, the avoidance of retconning cemented the sexist nature of many series. I don't think that's by accident. By far, the removal of overarching sexism is the only parts of these stories begging for a retcon. This sort of sexism, so pervasive in SF&F, is ancestral to face of sexism today. If the authors had stood their ground and publicly retconned for inclusiveness, I think that SF&F would have been well served decades earlier. But who knew that it would all come to this? Nobody had a roadmap of the future.
From here, I see no lessening of strong continuity. Indeed, I see it growing stronger, but I also see that some media have figured out how to loosen continuity when needed. I think that books need to learn from that. More than anything, writers need to go where the best stories are, and if that requires changing a few facts, then those facts should change.