Your way is anti-creativity. If metaplot is required to justify any changes to the setting than a) if your setting has a perfect starting scenario (Eberron), then you have to abandon that perfect starting date and explain what else changes as time progresses (technology, politics, new NPCs) and b) work backwards from the changes you want to make to the justifications you have to make to implement that change.
Like, for an example of why I think metaplot is a plague on D&D settings, the Dragonborn. In 4e WotC wanted to add Dragonborn as one of the core races of D&D. Which in a roleplaying game called "Dungeons and Dragons," anthropomorphic dragon people being a major player option just makes sense. However, they didn't want to have all Dragonborn be created by Bahamut (like they were in 3.5e) and needed a justification for why there's suddenly all these Dragon people walking around the Forgotten Realms now. Ideally, they should have just snapped their fingers and said "Dragonborn exist now, they've always existed." In which case, most people would judge the new race based on how interesting they were.
But, because the Forgotten Realms has this terrible tradition of using metaplot to change the setting in extreme ways, they came up with the idea that Dragonborn come from
a continent on a parallel universe that was shunted onto Toril because the God of Lies murdered the Goddess of Magic (which is like the 3rd time she has died). And that's where Dragonborn come from, kids. Which is unbelievably stupid. Technically it's "justified/allowed" according to your guidelines for what makes a good "retcon," but that doesn't make it
good. And a lot of people hated this justification, and dismissed the race as being terrible because WotC's lore for where they came from was terrible.
So, Goliaths are a core player race in the 2024 Player's Handbook, but they don't really have a big role in Eberron.
There have been minor references to them before and
Keith has proposed a few ways you could include them in your Eberron, but they are not a core Eberron race. Let's assume that WotC comes out with a new Eberron book in a year or two, and decide that all of the core races of the PHB need to have a major role on Eberron, but also assume your approach to worldbuilding, so they say "100 years have passed since the original Eberron starting date, and in that time a continent from an alternate universe has teleported to the setting and it's inhabited by Goliaths," I would think that's idiotic and proof WotC didn't understand the setting. I wouldn't buy that book and if any of my players asked to play a Goliath, I'd come up with literally any other justification for why they exist in the setting. Because the way Dragonborn were introduced to Eberron was infinitely better than this. But progressing the timeline in order to introduce a new race is stupid and ruins the feel of the world.
Which is why I think your approach to lore, where every change to a setting has to be justified with metaplot, is really bad. Just an awful approach to worldbuilding that would ruin so many of the settings that I love because you don't like retcons. It's way better to wave your hand and change the setting to how you want it to be. If the change is good, great! Future books in that setting can keep using it. But if the change sucks, they can just wave their hand again in the next book and ignore that it ever happened.
You consider consistency to be more important than quality. I consider quality to be more important than consistency.