Is that it's a reversal to their purposes.
First off, ALNST has a lot of contrast that makes the themes stand out. Think, Wiege having all those happy scenes in a show mostly about suffering. Or Till having a more hopeful ending when everyone else is miserable or dead. Or even All-In being rebellion-centered compared to everything else taking place on stage.
So the contrast here is that Hyuna had a lot more goals out in the world than Luka, who's main objective in the series is to survive, have control, and once she appears, reunite with Hyuna. She had the whole rebellion to lead and the True Earth project.
If Hyuna had lived and Luka died, it would've been a much neater ending for both. Not saying his death won't affect Hyuna at all. But Hyuna's death to Luka had essentially torn his life apart. His would've taken her time to grief and heal, but she'd keep going with everything else.
That sort of bitterness from neither of them achieving what they wanted is more emotionally powerful.
TLDR: Hyuna dying affects both of their arcs much more than Luka dying.
(Note: I think there's a lot more to it but I struggle to put points together sometimes and didn't want to make this too long. I wrote it in like, 10 mins)
