This ignores situation entirely. Dom is way more attached to Darlene than vice versa (Dom has zero personal attachments, so of course she can't get over someone she slept with). Darlene has been struggling with crossing lines/being a good person and desperately doesn't want to get to where she was (killing the lawyer in her house/people dying) again. Dom said the exact thing to her that she never wanted to face, that she was a horrible person destroying good peoples lives. Darlene was trying to find redemption via doing everything she could to undo what she did to Dom, Dom was struggling with the fact that the only person in the world she's not related to she feels ANYTHING for is her Alexa and the woman that ruined her life. Their bond is fukked up but it's strong.
And they did NOT run away with each other. That's the point. The only reason they were going to was the Dark Army. In reality Darlene wanted to go because she's co-dependent, thought she was saving Dom's life, and finally found an out from the downward spiral she'd been in dealing with the dark army and her morality. Dom was only going because she thought it was the only way to save her family.
Once that got taken away look how quickly the idea of "them" fell apart. Darlene in the bathroom realizes she can take care of herself, and Dom on the plane knows Darlene isn't coming but she needs to go anyway, because the life she was stuck in was worse than death.
Dom's family is fine. She's in no danger, but she has nothing to do back to than her life which was just loneliness and misery. There's no reason to stay.
Ditto for Darlene. Dom is safe. There's no more fear of blood on her hands, and she's not afraid of being alone (being with Elliot is basically being alone), but her family is back there and needs her help so she's staying.
This episode was them processing their shyt and growing as people. One has no more reason to fight (started the episode thinking she did), whereas another realizes she has it in her to fight (started the episode feeling utterly finished).
It's like you missed everything family I don't know what else to tell you