Reading your post about the Piltover vs. Zaun arc in Arcane s2, I wonder what your thoughts were/are about the alternate universe Heimerdinger and Ekko visited? I felt "the death of one child" leading to to the utopia that Silco + Vander were aiming for was highly implausible (no *particular* real-life counterpoints come to mind) and wishful thinking from the writers.
I think it can be argued that the division point could have been somewhere else or there could have been more than one. Heck, the real division point could be that Oriana never got sick so Singed/Dr. Reveck never developed Shimmer and that's the reason why Silco was open to reconciling with Vander after Vi's death in the first place. If you want to believe that more happened or that other factors were also in play, I don't think the alt universe explicitly denies it.
That said, I do think that within the story, it is clearly meant to be Vi's death and the subsequent butterfly effect that led to the "utopia" such as we see it, like you say. We can speculate but it is very heavily implied that it led to Vander and Silco reconciling but, more importantly to my eyes, it led to Hextech being stifled before it could ever fully blossom, which is in keeping with Arcane's larger themes, particularly around the Shimmer vs. Hextech arms race and its larger place in the Piltover vs. Zaun narrative and juxtaposition.
It's not just the possibility of a Vander/Silco reconciliation/marriage (which is more central to S2 as a "what-if" than S1) but really the sort of monkey's paw impact of Hextech (it's not a gift, it's a curse), the arms race it set off with Shimmer in the undercity, the hyper acceleration of Piltover society away from its previous industries and thus away from its need to support the undercity in any way, because now they had magically-enhanced international trade instead and therefore turned a blind eye to Silco's takeover down there, etc etc.
Honestly, I don't find it all that implausible or naive though that the "small" occurrence of Vi's death led to this because S1 of Arcane, literally every single second of it, every single scene, is structurally answering one question, "How did we reach this point where Jinx, who was once a traumatized and seemingly insignificant little girl from the undercity, fires this rocket at the Council thus changing the fate of everyone in the city, and what are all the points previously where it could have been prevented?" And S1 is so tightly written you can actually choose almost any given scene and see that if it had gone the other way, Jinx and the rocket wouldn't have happened.
And that exact layering of butterfly effect scene after butterfly effect scene in S1 goes all the way back to the start chronologically, the actual inciting incident that pre-dates Vander and Silco, which is when one child did not die: Jayce in the snow, when the Mage rescued him and his mother, which brings the story full circle in time as Jayce eventually goes on to create the Mage through magic, who will one day go back and save him and spark his love of magic.
So again, I don't find it naive or implausible or out of keeping with Arcane's themes that the death of one child can cause such a huge ripple effect (and there are still signs that not everything is perfect in the alt world, it's just a kinder place by comparison to canon) because the canon world is based off of the survival of one child in a situation where he realistically should have died and the entire plot and question central to S1 is about the incredible impact one once-disadvantaged child (Jinx) can have.
I think the themes the writers were trying to speak to were less, "Entrenched societal evils can be overcome by one small change an a community coming together over one child's death." And more, "By every death in the world we are changed and even the smallest person can change the course of the future in unpredictable ways, some good, some bad, because no one is an island and everyone is significant in ways you can't imagine."
Oh, and I will counter that no one death has ever led to massive societal change, when at the time many people credited the launching of the Arab Spring to the death of one young man, Mohamed Bouazizi.
