Austin doesn’t reject Eunice because she failed to say “I love you” in time. It feels much more like a turning point where he begins to grow up and accept the reality of his situation. He understands that taking the USB to the police isn’t some heroic solution and that it would probably put Ash in danger, and given how powerful Park is, it might not even change anything.
At the same time, the fact that he has a baby on the way forces him to think differently. What he feels for Eunice is the "honeymoon phase love" or the early stage excitement, it represents something idealized almost like a fantasy. In that moment, he starts to see it for what it is and realizes that life doesn’t work like that. You don’t always get to choose what feels right, sometimes you have to choose what’s responsible. And I like that Ashley even says that and It is actually mentioned many times throughout the show, "we at least know each other shits and who's to say after you find out what Eunices problems are you won't decide to leave" or something in similar fashion. Long lasting relationships require sacrifices and require you to choose if you want to accept other peoples problems or not and living with those choices.
We see a similar pattern with Josh, who sacrifices himself for Lindsay and then has to live with the consequences of that decision. Both storylines reflect the same idea: people are constantly trying to figure things out and chase happiness, but reality forces them to adapt.
I just feel like Josh and Lindsay finally in the end grew out of it, accepted the punishment and the nature of normal everyday life, which they were kinda wanting in the first place but their struggles always kinda pulled them back in. Great representation of that is losing their dog, they slowly start to realize that nothing really matters as much as love for each other, just the pain was too strong at that point so they couldn't really stay together, they needed time away from each other yet they never lost love for one another.
The show suggests that growing up isn’t about finding perfect happiness it’s about learning how to live with the choices you make, even when they hurt. It’s about accepting pain instead of escaping into illusion.
So Eunice doesn't have to be "dead" but Austin is not happy in his current situation and is probably living with regrets of what could have been. In the end he understands that they cheated through for their current jobs/positions, probably similar to how Josh and Lindsay got there in the first place, became managers and are bound to repeat the same cycle of constantly chasing a fairy tale while also being at Park's mercy, technically slaves, not really an ideal life.
In a way Josh and Lindsay got a happy ending, Ash and Austin replaced them.
If my understanding of the show is correct than hats off to the writers, this is some genius stuff about life.