But the WoL has to move forward throught their despair. Unlike us who knows that they wont just kill all the scions off, the characters in the story dont know that. Which is imo shown when all the answers to the Twins are about them not going through with it. For the WoL this was probably their highest moment of despair they ever felt. No matter what we feel about the characters, the WoL that the devs wrote cares about them. The WoL does not know that they will be back at the end. They still go on, just as they did with every person they lost. They may be sad, they may remember them but they always go on.
And at the end their iron will to continue summoned Emet and Hythlo. And by understanding Meteion they summoned the Flowers and thus Hope.
I find it a bit sad that seemingly for so many only a main character dead means much. Yes we did not lose any of the main cast but we still lost others. After Ahewann's death I was on the edge of my seat when we had to help Matsya. Especially since they had no problem with turning a kid into a beast. I cried when he tried to to give himself courage but failed to do that. And I was shaking a bit after he finally got rescued.
And honestly I found our final days to be worse. We were only at the first stages but we already learned that people bascially stopped to exist once they turned. That for me is somehow the worst. To never be reborn again. Never the chance at another life. While you devor and kill those that you hold dear. For me at least that were pretty high stakes.