First time posting to BL so let me know if this is the wrong place to post this, but I had a funny theory that I thought I might share.
In Saber Wars II, its established that nearly all of the denizens of the universe are servants, except a few handful of Masters. All of citizens were turned into servants long ago in an event called the Universe Update. If I remember Saber Wars correctly, none of the servants in the Servantverse ever truly die (main examples being MHXA and MHX at the end of the event). This is very similar to the Third Magic aka materialization of the soul.
The Type Moon wiki says (unsourced so take with a grain of salt), "Due to a phenomenon called the Universe Update that appears to have imposed the effects of the Third Magic on a galactic scale billions of years in the past, all but a handful of humans have become Servants in this universe, with only seven Masters left in existence." [1]
The theory I have is in this parallel world, Amakusa Shirou was able to enact his plan for Heaven's Feel at the end of the event of Fate/Apocrypha, and wasn't stopped by Sieg. His wish to spread the Third Magic to sublimate all life ended up turning all of humanity into servants (except for the few masters). It's the closest any character in Nasuverse was able to enact a universal Third Magic, which is why I specifically point to Amakusa. The end of the Great Holy Grail War was the moment of the Universe Update and the creation of the Servantverse.
Of course, Saber Wars is the definition of a lore-breaking joke event, but I think it would be even funnier to imagine Amakusa's dark and serious master plan resulting in Mysterious Heroine X and co. Just wanted to share a fun theory!
Wiki Sources
[1] https://typemoon.fandom.com/wiki/Servant_Universe
[2] https://typemoon.fandom.com/wiki/Third_Magic