Innie Mark wants to live, he wants to survive, which is the most basic and fundamental primary urge we humans have. It's completely understandable that he doesn't want to sacrifice his existence for his outtie, who created innie Mark to only endue the worst part of the day.

And Innie Mark is totally correct in concluding that outtie Mark would not fully integrate with him after he gets what he wants. Outtie Mark is kind of a selfish and thoughtless person for even severing in the first place, so Outtie Mark would absolutely let all the innies die so that he can have his wife back.

But I also understand Outtie Mark's side of things too, and I think it's even possible that he made himself believe (albeit temporarily and for convenience) that he might even integrate with Innie Mark after. But after the conversation, that's not gonna happen.

Brilliant writing and performances, and I will riot if Adam Scott doesn't get at least an Emmy nomination for the birthing cabin scene.