Yeah, I thought of this, but it's not necessarily an irreconcilable issue.
First of all:
Doesn't this work against it being tied to royal succession? How often is the throne succeeded? I have to imagine it's less often than every 5 years. That would make you think that they could stock up on them if it were the case. But if it were essential for reproduction, then they would basically be required to keep consuming them to keep the population stable. Of course, maybe the fruit isn't able to be preserved, and that's the reason why they'll only be able to have one (royal?) wedding?
Also:
We don't really know the logistics of the plant aside from that it bears fruit once every five years and they can only procure one of them at the moment for the saints and princesses. Let's say that the fruit really is essential for reproduction in this world. That would likely make it one of the most valuable resources in the world. Maybe there are multiple plants, but the ownership of them is distributed across different towns/communities. Imagine the royal family marches into your town and says, "We're taking your magical fruit, you won't be able to have more children for a 10 year gap." That would probably start a revolt. It could be that the royal family has their own Autore plant, and then each town also has their own Autore plant. This would explain how the population is kept stable and also why they aren't able to procure more than one. (It would also keep the possibility of letting both sages marry the princesses if they happen to find another plant/procure a fruit through some means.)
As for the population logistics:
If one couple gets married every five years and has a child every year, in a 25 year generation, 75 children could be born from a single plant (double this number to 150 if both spouses can get pregnant simultaneously). It wouldn't lead to exponential growth, or huge numbers like we see in the real world, but it could theoretically be enough to sustain small populations. Especially so if there are multiple communities each with their own plant. We don't really know that much about the size of the world or anything, so it's maybe not entirely unfeasible of an explanation.
And yeah, I might be reaching a bit. I guess the reason I latched onto this idea a bit is that it felt like this whole thing about reproduction and lineages has felt like a mystery that the author's been dangling in front of us in the background this whole time, and the chapter where it's finally explicitly mentioned by a character (Tsubasa) also happens to be the same chapter where we first hear about this fruit? I don't know, it felt deliberate to me, but I guess we'll see.
So I'll preface this with two things:
- One, I applaud your breakdown and reasoning.
- Two, I was a moron and think I'm misunderstanding the purpose of the Fruit in the first place.
I'm now going off the idea that the Fruit is specifically used to allow Otherworlders (read: Saints) to have children with the people of this world. I'm guessing there's some physiological distinction of the women of this world that allows them to have children that wouldn't exist where our two Isekai'd Saints come from.
- -
I'm not going to posit on what it is, I'll just say "Fantasy Say Gex Preggo Ritual" and leave it at that.
But I bet it
is completely "natural" and doesn't require specific rituals or potions or something, because even if Magic existed alongside humanity since the beginning here, not every person could use magic, and requiring outside help and/or manufactured components means that it would be very easy to "lock out" certain people from having children. Nothing about the worldbuilding thus far suggests that amount of prejudice or oppression exists, so I'll steer away from that idea for now for the sake of efficiency.
*If every person
can use magic, then maybe there's some inherent magical element to every person here that aids in procreation. That could be a component even independent of it being a "404: no men found" world, but that's a separate topic at that point so I'll stop here.
- -
But the Fruit has to be consumed by the Saints, as it has specific properties that instill that physiological characteristic in the Saint that allow them to have children with the Princesses once they marry. (Since the Saint always marries the Princess and they both go on to become the ruling couple, unless I missed something super important.)
And maybe this plant
is super rare/bears fruit super infrequently, because it is the product of massive magical consumption and rituals to grow the plant/blossom/bear fruit. Maybe those are natural, maybe they're cultivated, but the amount of magic needed makes it an arduous, time-consuming process that averages out to about 5 years for a single fruit.
Alternatively/Additionally, the plant only grows in a specific region that is hard to get to, but otherwise kept safe; maybe guarded by some entity that makes continuous harvesting too difficult to attempt except once every half decade (dragon or some monster that goes to sleep every 5 years, and that's when you can get past it to the plant, or something).
In any case, this results in only having one Fruit for one pair of Saint/Princess for the time being. Usually there's just one Saint, so it's not a problem; but HOORAY PLOT so now we're in this mess.
But I think that more efficiently satisfies the purpose of the Fruit, how the world's population has persisted with minimal issue up to now compared to the expressed rarity of said Fruit, and why it would be so important to Royal Wedding Ceremonies, specifically.
