There's actually a fair bit to unpack here in the ending scene.
First: This isn't the same Chihaya we've been following. The one with the sweater and the long hair. At first I thought maybe she was somewhere else and she'd save Ouka from the short-haired one, but then I remembered what this Chihaya said. If all Chihayas are dead and there's no one to chase Ouka then there is no game and she wakes up. So, we know that she is likely dead.
Second: A new game means a new set of Chihayas. It's very possible that some of them will likely want to help Ouka escape the coma, so ideally Ouka simply needs to wait until another one comes around. However, that could take a very long time, and by the time she comes out of her coma she could be either very old or simply dead of old age, assuming she didn't just outright die this time from going into a coma at a bad time.
Third: There is a new Merrick on this Chihaya's shoulder. Merrick has always done what they could to assist Chihaya, but their ultimate goal was to help Ouka. Since Ouka wanted to maintain the game then that was their mission. Given that Merrick/Ouka reacted in distress to Ouka being locked in the train car and, more importantly, that everyone is reacting to her like she's still the boss, then it's likely that this Merrick will also help Ouka. Perhaps Merrick takes the bullet for Ouka, or manages to move Chihaya's arm to the side so she misses. Alternatively, perhaps this is all a ploy by both Chihaya and this new Merrick to scare Ouka into staying awake by forcing her to play one more game before waking her back up, but given how obsessive some of the Chihayas became it's far more likely that this Chihaya simply lost her sanity at not being able to be with Ouka.
With all that said, I'd like to think that Ouka will be able to escape eventually before she dies, and when she does she'll realize that staying in her head like that is too dangerous. As lovely as it is to meet the literal person of your dreams in, well, your dreams, it's not a good thing to be so engrossed in your dreams that you are unable to live in the waking world. Dreams can be nice, and mayhaps some of them can become real, but it's unhealthy to dwell upon them too much.
