A Wibbly Wobbly Timey Wimey Citation At Last

[Iscah]

The Alexander storyline already introduced the concept of countless timelines:

'Alexander dreamed all the realities imaginable - all the realities mathematically computable - and in the end, reached a single, logical conclusion. It would change nothing, and erase itself from existence.'

...

'There was but a single time Alexander was spurred into action - not to change history, but preserve it. The summoning of the colossus, and events that followed, had potentially disastrous consequence for our reality. Its fabric strained to accommodate an infinite number of potential futures separated by nary a thread.' (Dayan, Lv. 60 Judgement Day)

The thing is, I don't see that as evidence of a permanent, ever-splitting multiverse, but the opposite – there is ideally only a single timeline, which Alexander is trying to safeguard both by calculating its best possible route and by ensuring that any time-travel meddling will form stable time loops rather than changing something and causing a split.

While the possibility of time travel is in play (I take from these quotes), the ideally-single timeline becomes strained, as each second brings new possibilities of it diverging off in new directions if the time traveller does something incompatible with their original future. But this is a very different prospect to it spontaneously splitting all the time, and as Dayan says, it is not the intended state of the timeline.

To go back to the explanation you quoted in part:

There was but a single time Alexander was spurred to action─not to change history, but to preserve it. The summoning of the colossus, and the events that followed, had potentially disastrous consequence for our reality. Its fabric strained to accommodate an infinite number of potential futures separated by nary a thread.
Were the wings of time to fall into the hands of the Illuminati, the repercussions would be dire indeed. History would be rewritten over and over again, each time bleeding the land of aether. And in the end, the colossus would usher in another calamity.
To prevent this tragedy─to preserve the circle of time as it had already been set in motion─Alexander sent forth a humble servant to do its bidding.
A clockwork coeurl, an eternal child, to gently nudge history back onto its proper path.

The "infinite potential futures" were an unnatural strain on the timeline caused by Quickthinx's scheme; preventing it brought time back to its ideal state, a single "proper path".

That path diverged later to bring about the Eighth Calamity timeline, of course, but if it was just a single split then perhaps that can be accommodated, especially when it becomes essentially a necessary loop-the-loop to build the Crystal TARDIS (the work of several lifetimes) and get it positioned to defend the other half of the split timeline from ruin, twice – the best possible outcome Alexander could hope for, indeed.

[Lurina]

I don't think the writers really have a plan with the time travel stuff. Alexander and Elpis were closed loops because they thought it made for a good story, and Shadowbringers wasn't because they thought it made for a good story. And now they're saying it's all an open ended multiverse where new timelines are constantly formed because that's a convenient way to dismiss the incongruence.

Despite what everyone seems to be concluding elsewhere, it feels likely to me the Milalla also travelled through time and Alexandria originates from prior to the 2nd Umbral Calamity, too - that possibility was explicitly raised within the narrative, and it's an hourglass, after all. I don't understand why anyone would expect adherence to any consistent set of metaphysical rules when Shadowbringers and Endwalker made it obvious this is all being written by the seat of their pants.

[Cleretic]

Despite what everyone seems to be concluding elsewhere, it feels likely to me the Milalla also travelled through time and Alexandria originates from prior to the 2nd Umbral Calamity, too - that possibility was explicitly raised within the narrative, and it's an hourglass, after all. I don't understand why anyone would expect adherence to any consistent set of metaphysical rules when Shadowbringers and Endwalker made it obvious this is all being written by the seat of their pants.

I mean, with that one I'd more caution an Occam's Razor situation than anything; it's not that it's impossible that the Unlost World is from the shard lost by the Second Calamity, it's that it's less likely than another possible option (that it's from some other shard that didn't get rejoined).

It's a 'why take a longer road to the same destination' kind of problem.

[Lurina]

I mean, with that one I'd more caution an Occam's Razor situation than anything; it's not that it's impossible that the Unlost World is from the shard lost by the Second Calamity, it's that it's less likely than another possible option (that it's from some other shard that didn't get rejoined).

It's a 'why take a longer road to the same destination' kind of problem.

With respect, that's true only from the perspective of a reader. It's "simpler" for the Milalla not to have time traveled, and for it not to be possible for a remnant of a rejoined shard to be left floating in the void insofar as that concept has never been established, but only in an in-universe sense where this all some real unfolding history.

If you view the final scenario as the product of a series of creative decisions going from the foundations of the premise to the specifics, it seems more likely they decided on having a lightning-themed group from the lightning-themed dead shard first, then just fudged the details to make it work with the planned metaplot and decisions from the other creative departments, like the appearance of the post-MSQ state of Living Memory.

I could be wrong, but...

[Iscah]

And now they're saying it's all an open ended multiverse where new timelines are constantly formed because that's a convenient way to dismiss the incongruence.

I've been saying for some time that I don't see an incongruence between the "usually stable loops but one instance of a split". Both can coexist with the right set of rules.

It makes sense to me that the setting seems to prefer stable time loops if it is possible for them to form, but a split can result if someone is time-travelling to the past and creates a situation that contradicts the future they came from, so the timeline splits due to the paradox.

Introducing infinite additional worlds is what feels like the incongruence to me. I was saying that when it was theoretical, and I'm sticking to it now.

[Lyth]

The "infinite potential futures" were an unnatural strain on the timeline caused by Quickthinx's scheme; preventing it brought time back to its ideal state, a single "proper path".

My interpretation of this was that at the moment of Alexander's summoning, the timeline could have diverged in infinite possible directions based off of Alexander's decision. For example, Dayan describes a future in which Alexander averts the Seventh Umbral Calamity by sweeping Dalamud out of the sky. It's not a singular bifurcation on a decision tree. Alexander had the power to change absolutely anything in the timeline in an instant. That's why the infinite 'potential futures' (i.e. a multifurcation) were separated by 'nary a thread'.

It's worth noting that Yoshi-p isn't necessarily invoking western pop-culture interpretations of a 'multiverse' when he discusses 'branch theory'. I don't think he's suggesting that all the Azem reboots are going to get together and make an ensemble film. Nor is he implying that 8UC Emet is going to lose his memory, get a cyborg arm, and then fall into a dimensional rift into our world. I think what he's describing sounds a lot more like the standard light novel 'What if' side story, where you evaluate the consequences of a specific decision against the events of the story. The point of the 8UC 'What if' was that humanity would find a way to survive even without the Warrior of Light, but the course of that history remains unwritten.

I think an important consideration in this is that significantly altering events in the past causes timelines to diverge. If you go back into the past and change an event, you are now in a new timeline and cannot return to the old one. G'raha has no way of returning to the 8UC timeline, for example. You can't really jump timelines the same way that you do dimensional travel. We have dimensional travel already, as each shard exists in the same physical co-ordinates but on a separate, displaced layer of reality (EE Vol 3, p.13). You are always continuously existing in one singular timeline, regardless of what you do. Changing an event, either in the past or present, just causes your current timeline to splinter off and bifurcate. The alternate timelines are specifically what you didn't do, and you have no real way of interacting with that.

I suppose a mathematical way of thinking of it is that your trajectory within a timeline is path-dependent, while spatial or dimensional travel isn't.