I think I get the message now. The zombies become unified, and then all individuality ceases. They form one being, one mind. This mind experiences all emotions and memories equally. This means it doesn't really experience any emotions. No solitude, no joy, no pain, no anger. Just nothing.
It is ironically just like being a self-accepted neet. Shutting yourself away and forgoing connections just to be comfortable. There is no difference between Hiromi inside that nest, and Hideo hiding inside his apartment at the start of the series. The Hive preaches connection and unity, but you can't be connected without there being people different from you. You'll just end up alone, a neuron in a massive brain.
This is what everything in the story has been leading towards. Hideo's lack of emotional response to Oda and his girlfriend. Hiromi's hate of people. The violence that people commit against each other. Oda's refusal to lose her sense of self. It's all about having the courage to exist in a world with people that you don't know. To form connections with others who are different from you.
I'm hoping Hiromi realizes this and chooses to exist with Hideo as separate people.