You are probably right. The next step would logically be the slow death of sex, lending itself to a dark story how everything turns boring eventually. I imagine ever more extravagant but futile attempts to get some of the old feeling back. Huh, I can't help it, somehow I can never imagine a bright future for those two, it eventually feels fake.
Edit: I recently reread Genocide, and I have a question that I somehow missed on the first reading. I will put it in a spoiler since it pertains to the last chapter.
Your second "Instrumentality" scene confuses me. You let Yui say "Sometimes not even the love of others can comfort us. Sometimes we hurt too much. And we break". Yet, she answers his question ("Should I give up on Asuka?") with "I'm saying you shouldn't let that happen to her". Is it just supposed to be a cautionary tale? Don't let things get so bad that they can't be fixed anymore? Somehow I got the feeling that I am missing some deeper meaning here.
Edit: Or is the message "Try regardless, don't give up"? Seems like I have hit a mental block here, it makes less sense the more I read it. So, bluntly: What was the message here? Besides the messages of: Do not neglect your own happiness entirely for somebody elses, accept death and loss, don't give up?
Edit 2: I think I get it now: If she doesn't want help, there's nothing he can do?