Why can’t Finny survive?

Emotionally Finny needs Gene too much. True, Gene needs Finny but at least Gene has the ability to move on. The thought of life without Gene scares Finny so much that he ends up shattering his leg again. The operation for this ends up killing him. Finny ceased to exist after his accident. He only carries on through being with Gene, moulding Gene into a version of his former self. Still, after Finny dies, we get the sense that Finny lives on inside of Gene. It's not a warm kind of "oh I'll keep Finny in my heart forever" thing. It's much darker than that. It's like Gene assimilates Finny into himself.