I really like how this series accurately portrays how relationships can fall apart. We just say "that's fine" when we don't really mean it. We don't know when we're lonely, and we find distance growing between us as time passes. Onee-san's coworker mentioning that it will lead to an impasse is 100% accurate, as people can only live that way for so long before something happens, whether that's a fight or infidelity in some capacity (emotional cheating is common to start with in this type of scenario).