She looks surprisingly upbeat for someone who's lived billions of years. Nice.
It is an alternate interpretation of immortality. After all humans tend to keep in mind only the few years just passed, in some works immortals are simply like that. Yes, they are around hundred, thousands, or even billion of years. But they actually remember only the last few (plus very special moments of their life).
But the characters of Touhou are capable of interplanetary and even interdimensional transportation.
In Touhou, the Sanzu no Kawa, Hakugyoukuro, Heaven, several of the Hells, etc. are all places you can just physically fly into and go wave and say hi to the dead.
Furthermore, in this infinite multiverse scenario, why is it no life exists anywhere but Earth? The Lunarians in general are extremely long-lived, powerfully magical, and could easily make themselves a rocket ride to another planet, and, given that they can terraform the moon, they can easily make any random rock habitable. Outside of the utter destruction of the whole multiverse, there should never be a reason for all life but the immortals to die out. The Buddhist reincarnation system functionally requires that, unless absolutely everyone started gaining enlightenment, there be worlds into which one could be reincarnated.
It's fanfiction. People make up and change whatever they want in the source so their fan story can work. I'm not defending it or anything. I think it's dumb, too, but what are you gonna do? Fanfiction is as fanfiction does.
Another thing, I think gensoukyo is probably some kind of weird time/space anomaly like the place from dark souls or the end of time from chrono trigger where time doesn't actually pass and it's connected to all time periods at once. The main reason I say that is because a letter written by Renko in the near future somehow ended up in the distant pre-barrier past with the 8th Hieda. If all that's true then this whole story is super dead in the water.
In such harsh environment, there were girls who couldn't die, no matter what.Houraijin: A pitiful sinner who has fallen into the Eighth Hell by drinking the Elixir of Eternal Life. YET!!!Dammit, the Earth's gonna end soon...ANDTHE IMMORTAL WHO SAW THE DEATH OF THE UNIVERSEWhat will become of me then...?
