@Tamerlane
I think you might want to give this a look, cause no one actually wants to live forever for many reasons:
https://mangadex.org/title/22440/jumyou-wo-kaitotte-moratta-ichinen-ni-tsuki-ichimanen-de
My interpretation. Most likely wrong but that's the beautiful thing about open endings.
So the reason everyone is "immortal" is because they are already dead? The people are most likely having a false understanding of where they actually are. Actually makes some sense "All the people who lived there at the furthest, furthest ends of time" (time = lifespan). So the people who "die" in this place are actually being brought back to life by the red fruit?