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?only see one path for that war, it doesn't mean others cannot exist.
One things to understand here:
Context. After the 4th grail war happens as it does in Zero, the fifth is 100% not survivable for him. You can say "Hurr but what if 4th changed," but that's unimportant, because what Avenger is saying up there isn't "COMPLETE AND ALL-ENCOMPASSING STATEMENT" it's "After the 4th grail war(as it actually happened) happened, Kirei can never survive the fifth."
See if "everything is possible" as Lantz decrees with his nerfed pro wrestler TYPEs and... Satoshi's entire birth circumstances, then even with the 4th happening as it did in canon, Kirei would have a way to survive the fifth grail war. Except Nasu, word of god, original author, directly contradicts him and his idea in F/HA.
Therefore everything is not possible.
Also, this thread is absolutely hilarious for all of the wrong reasons, my sides literally split in half, typing from hospital