The look he gets for the observation about duty is somewhere between fond and rueful. That certainly isn't something new about Herian, it seems.

But instead of diving in there, she considers his second observation, taking a drag as she does. "...yeah, probably. I mean. Part of me thinks we can't really know the ways we'd both be different if I'd just been here the whole time instead of vanishing. Like, I do think you're right, some of the fault lines were here even before I went. But also she lived the stretch without me. And I came back with new memories, including getting way more serious with my partner from back home, and even knowing that Cosima isn't me, exactly, it's... Someone you're with leaves a mark on you, too, even when you don't consider yourself with them anymore."

A shaky laugh. "I mean, the way you're definitely right is that I think it would have been a disaster if we'd tried to work it out just out of some sense of obligation to our earlier promise. I don't think we were in real danger of that, though." Herian, her general feelings on promises aside, had been the one to leave. If she hadn't, Cosima suspects that Cosima herself would have stepped away instead. Her self-preservation instinct is certainly that strong.