It's 2019 now lol. I wonder why I hadn't read this piece of art a long time ago.
Anyway, I think this is a good ending, and not in a bittersweet way. It's true happiness, and totally gay, uwu
Mr.Chief said something in the end of chapter 8: "You don't realize just how much Praha loves you, in her own robot way". Soooooooo, as a good father, he would just tell Chika to just marry her already lol.
Still at the end of ch.8, Chika said something along the line of "please marry me". Although a bit of a joke, she could very well mean it.
Also she came to a realization that:
1) "As long as I'm spending my time in a way I enjoy, as long as I can feel that supreme happiness"
2) "Now I realize, I wasn't sad just because Hiro-chan isn't human, [...] I've learned that even when you are in a relationship with a human, it's the same."
3) "That's why I can now trust in the feelings in my heart. I love her most of all."
We can conclude that Chika has finally accepted the fact that she loves a robot, and that her lover being robot or not doesn't matter.
Also, in the extra chapter Chika even decided to buy rings.
Additional info: Remember a guy who said "So you're the real thing... I see." to Chika? It can be infered that Chika and Hiro came straight to the HQ to greet the old folks. And right there Chika asked Hiro about her "birthday" and about buying rings. So it's safe to assume that she had sorted her thoughts and feelings beforehand and was on her way back home to find Hiro.
Now comes the bittersweet part, that is, human (and Chika)'s lifespan and aging versus robot's almost immortallity.
Chika never dies, or more accurately, not in the near future - she could possibly live for a few centuries more, who knows? As someone already gave an explaination above, please read the posts. And now, with just one sentence:
- Are you happy if you've got yourself a life partner that 1) is always beautiful and 2) never stops loving you regardless of how old, ugly you are?
FOR THE LOVE OF YURI GODDESS, I WILL TREAT YOUR ANSWER AS A "YES".