In one of yoshi-p interview he stated that they don't want the players to feel like they wasted investment on the npc and the world (eorzea) after endwalker. Going by this, I think they won't kill a lot of of major npc. Perhaps one of the scions will die, two of they are feeling brave. Getting rid of npc doesn't always mean they have to be killed though, so there's also a chance that the scions will disbanded and everyone will go their own seperate way.

To be honest, one of the annoying thing with ffxiv is their reluctance to kill popular characters, especially one that is fans favorite (with the exception of haurchefant, though I wonder if he's that beloved before he died or not). So yeah, I don't expect them to kill y'sthola, g'raha, the twins (especially alisae), and tataru.

I also thought the scions as my WoL's family because I like them. Has been since the start (arr). But after the mess that is 5.3, I don't know if I care about them anymore. I mean, I definitely care about thancred and alphinaud (I'll be really sad if they kill thancred), but the rest? Not so much. Two other member that I love is minfilia and lyse, but they're not in the scions anymore. At this point, I just want to be done with them and waiting for my WoL to have their own adventure and meet new npc that I hope are more varied than now (please no more female character which is just another y'sthola/alisae, I'm sick of them).