Potentially Controversial Take: I think the writers made the right call in not having Arendelle's castle be destroyed at the end of Frozen II.
Now why would I say this? Well, for a few reasons actually.
The first reason, and yes I am aware this does admittedly sound a bit cynical to say, is that i don't think destroying it works from a Marketing standpoint. The Arendelle castle is iconic imagery for the series, and I feel whatever castle that wouldve replaced it, would never be as iconic as that castle is. Removing it from the equation would just be a waste, and yes, I know Frozen II was supposed to be the final installment but even so, its still no excuse to justify destroying an iconic location in the Frozen series.
The second reason is that I feel like it would against the films themes of Cultural Erasure. Its clear Arendelld has a history of being a Viking Settlement. I.E. a people and culture that also had to deal with Colonization and Assimilation when Christianity was introduced to them. Now it is debated whether or not it was forced onto them or if they willingly accepted said practices and beliefs, but the point is is that Norse Paganism was demonized and nearly erased, just as Sami peoples culture and way of life (who the Northuldra are Supposed to be based on, key word being supposed to btw).
So i feel having the castle, that more then likely started off as a Viking place of worship (if the real life structure its based off is anything to go by) and has information containing Viking history and there beliefs, assuming it wasn't destroyed, would effectively be erased by the waves (no pun intended).
Also, before anyone even says "Runeard was the founder and first king! The Books say so", Jennifer Lee has already stated that she omly considers the movies and shorts canon, and on top of that, the fact that the concept art for F3 implies Norse Mythology is involved, tells me that they will likely abandon that idea in favor of Arendelle starting as a Viking settlement. As for the Junior Novelization of F2 that also mentions Runeard being the first king...well I mean, Novelizations aren't always accurate to what happens in the films. For example, the Novelization for the first Sam Raimi Spider-Man film features references to other Marvel Heroes that Sony didn't even own the rights too.
The third reason is that if the idea is to bring balance between Nature and Humanity, then I feel destroying the castle, and by extension the rest of Arendelle, i feel would make things even worse. Because you'd have to get the materials to rebuild everything, which will likely cause alot of environmental damage (I.E. cutting down trees for wooden homes and mining for rock and stone to rebuild the castle, which will more then likely cause soil erosion) and the people would likely be left without homes for a extended amount of time. Which would not be very stable. Finally, i think if the castle was destroyed, it would send across the wrong message that the past should be ignored and erased. We need to learn from the past, and simply destroying it runs the risks of repeating the same mistakes.
Especially since not everything about the past is a lie, not every historical figure was evil. Yes, History being darker then expected is a sad reality most of the time, but not always. Runeard was an evil person (unless the third film tries retconning that or tries giving him a empathetic/sympathetic backstory, though honestly i kinda hope not, because that be really stupid), but just because he was bad, doesn't mean everyone who came before him was. Destroying Arendelle's castle feels like some Kylo Ren from The Last Jedi levels of logic of letting the past die. Even though thats not the answer, learning from the past is.
So in short, this is why I think the writers made the right call in not letting Arendelle castle be destroyed at the end of F2.