Hollow Ataraxia. Within the dream world Avenger creates he can generate anything no matter how small the chance of it happening is. Kirei is not in HA because he cannot survive after the 5th HGW has ended. If there was a chance of Kirei surviving the HGW than he would be in HA, but he isn't .
"Upon surviving the 4th war, that man was promised certain death on the 5th"
-Avenger
Like Lantz said, what if there is no Grail War, or the 4th War is different?
Don't tell me you actually believe that with quantum physics everything is possible. Quantum physics is heavy on probability, and only things possible can happen. Impossible cannot happen. Unfortunately, the belief in quantum physics' omnipotence is the major superstition of 21st century which needs to be rooted out.
It is true that Many-Worlds theory only allows the possible to happen, but Quantum Theory
also says that there is a (very) small but non-zero probability of you walking through a wall, so it allows for a
lot of weird shit. Further, because of the Chaotic nature of the world, a hell of a lot of things can change through quantum events (things like the weather, genetic mutations and possibly mental processes are all likely affected by quantum events in some way). Now, obviously some things can't be done. No amount of randomness is going to make Shirou fundamentally evil (without making him "Shirou in name only", anyway) or cause a person to do things they normally wouldn't do (although it can, of course, change who the person is), but you can get a hell of a lot out of it.
Of course, the vast majority of worlds would be ones in which the canon characters simply did not exist in their current forms, but somewhere out there would be worlds with a very different history where those people still somehow came into existence.
Unless Saber destroyed the Grail that will not happen. If Saber does destroy the Grail but Kirei is still soaked his life support from the Grail will still fail which will kill him in time(about the time of the 5th war). If she destroys the Grail before that happens then he will die from his wounds.
Sorry, I think you're being way too rigid here. There are certainly going to be universes out there in which the Grail Wars stopped a long time ago (indeed, Prisma Ilya is one of them, IIRC).
Kinoko Nasu, creator of the canon work from which you draw your ideas and write your stories about has stated, in the canon work, that Kirei will not survive the 5th grail war. There was no possible outcome for him to survive it.
Yes, given the assumption that the 4th war went the same way (which is necessary for HA since changing the 4th war changes the characters in the 5th). But, there is no reason to assume the 4th war cannot change. After all, the 5th war has 3
very different paths (minimum), so why shouldn't the 4th war be the same? Just because we only
see one path for that war, it doesn't mean others cannot exist.
