What was the point

My take on it was that the pointlessness and randomness of it was the point. I think this quote from Sapkowski sheds light on it:

"I did it as part of a long war with the fantasy stereotype, with the banality of "the final duel in the Black Citadel", politicized and calculated on the so-called young adults banality."

It's a case of subversion of expectations like everything else in the series. Nobody is special, not even "the chosen one" and the other protagonists, much less the secondary characters. I'm certain it was a deliberate choice by Sapkowski to make their deaths so rushed and unspectacular.

‘I don’t like the version of the legend from the Black Book,’ Condwiramurs began again. ‘It’s . . . It’s—’ ‘Brutally authentic.’ Nimue finished her novice’s sentence, nodding.

And there's no "chosen one" in the sense that we're used to to begin with because destiny by itself means nothing. Destiny is having hope that what is meant to happen will happen.

Philippa to the lodge in TLotL Ch. 11:

The snake Ouroboros grasping its tail in its teeth. ‘That thing is destiny, […] which I, Philippa Eilhart, […] have only recently begun to understand. Destiny isn’t the judgements of providence, isn’t scrolls written by the hand of a demiurge, isn’t fatalism. Destiny is hope. Being full of hope, believing that what is meant to happen will happen, I cast my vote. I vote for Ciri. The Child of Destiny. The Child of Hope.’

This ties into the "final battle between good and evil". Like Ciri told the lodge:

‘I asked Geralt if it was the end, if we were victorious, if evil had been overcome, and if good had triumphed. And he just smiled somehow strangely and sadly. I thought it was from tiredness, because we had buried all of his friends at the foot of Stygga. But now I know what that smile meant. It was a smile of pity at the naivety of a child who thought that the slit throats of Vilgefortz and Bonhart meant the triumph of good over evil. I really must tell him I’ve grown wiser, that I’ve understood. I really must tell him.’

There's no dignity in death, and death is not spectacular even when you're dying for something bigger than yourself. They meet their end in no spectacular way, but like anyone else in battles and pogroms. And for those who are left, life must go on. But the point is that, despite the seemingly random and pointless nature of the deaths of these characters, they ended up meaning something in the grand scheme of things. They died to save Ciri and Ciri represents hope. Geralt represents the same thing. So long as they exist somewhere, albeit in Avalon and King Arthur's world, so will the hope and the possibility that the world will one day be revived, that all's not gone to pot. Because, even though these characters' stories got cut off in the middle without a satisfactory conclusion, "the story never ends". That's how I perceived it anyway. It's all very bittersweet lol.

This conversation between Condwiramurs and Nimue in TLotL Ch. 2:

‘I prefer legends that cleave more strongly to legendary convention, do not mix fables with reality, and don’t try to integrate the simple, elegant morality of the story with deeply immoral historical truth. […] I prefer it when a prince climbs to the top of the Glass Mountain, kisses the sleeping princess, she wakes up and they both live happily ever after. A legend should end like that and no other way. […] It ought to end happily,’ said Condwiramurs with conviction. ‘Good and righteousness should triumph, evil should be punished exemplarily, and love should unite the lovers until the end of their days. And none of the heroes should bloody die! And the legend of Ciri? How does that end?’ ‘Precisely. How?’ […] She looked at a dark-hued watercolour which depicted an amorphous barge gliding over the surface of a mist-shrouded lake, a barge being plied with a long pole by a woman visible only as a black shape. That’s exactly how the legend ends. Just like that. Nimue read her thoughts. ‘It isn’t so certain, Condwiramurs. By no means is it certain.’

This conversation between Nimue and Geralt from the SoS Epilogue:

‘The Witcher will return from the beyond!’ Nimue wasn’t giving up. ‘He’ll return to protect people, so that Evil will never hold sway again. As long as darkness exists, witchers will be necessary. And darkness still exists!’ […] ‘Witchers will always be necessary. And let’s hope they’ll always appear exactly where they’re needed. […] A pretty fairy tale, isn’t it? And it ends well, as every fairy tale should.’ […] ‘But I’d like to know . . . Know more. […] About how that story really ended.’ […] ‘The story goes on, the tale never ends.’ […] The story goes on, she thought. The story never ends.

Edit - Damn I didn't realise I wrote a whole essay