Mistaking longevity for experience is one of the cringier aspects of my own history. The longest failed relationship I've had was 4 years. Before that, 3 years. What did I learn in that time about love, intimacy, cohabitation? Not much. Most of the learning happened in the gap between the relationships. Once I was committed to someone I was safe from the hard lessons - wounds open but sterile - caught in a fond stasis.
Anyway this was meant to be the most dangerous moment, their shadows black against the wall, where Hanna and Will could really screw this up. I guess you (or I!!) could argue that they already have, being intertwined in so many ways. How long CAN this be sustained? How long can you be codependent on a person with this kind of baggage? I mean, potentially forever. But the potential for misery is as deep as any affection.