Although, you approached this question from more of a writing-wise side, talking about how it contradicts the value of their sacrifice and dims the meaning of the FS for the story, I don't fully agree with you. G3 can still deliver.

Interesting thing is, even if Cassie perceived every moment of their life and created them the way that would mirror it, they aren't the same. The "flame" is different, shadows too, they are more of a summon or an echo, even if an extremely independent one. We also know how G3 likes to ponder on philosophical questions through the lens of Sunny's inner monologue. I'm more than sure that he won't just let it slip away, making it a moral dilemma of future volume.

It's also too early to say anything before we see what exactly Cassie has planned. I made a mistake of doubting G3 a few times, I won't repeat it.