So it's established in FFXIV that reincarnation is the process of a soul going to the Aetherial Sea when they die, being washed of their memories and then being reborn as a new person. Presumably the population of the Source doesn't have to stay constant, maybe there's a means of creating new souls or maybe there's just a really big surplus.
In Alexandria, though, when a person dies of natural causes they capture their soul using a Regulator where it gets sent to Origenics to be wiped. People can buy souls, and that it seems to be common practice among the average person to keep extra souls in their Regulators so they can revive in the case of accidental death. Presumably this means in an accidental death, the system doesn't or can't absorb the soul that dies, and it reincarnates the normal way. The question is, if the Shards aren't connected to the Aetherial Sea in the Source and a significant portion of the souls Alexandrians do have are kept as life insurance by the already living population, how are children born with new souls?
Considering the Alexandrians lived for generations with no reason to leave Everkeep's barrier, I doubt there are many accidental deaths, so most souls do go to Origenics. No idea what kind of population size Alexandria had before they retreated inside the barrier but it's reasonable to assume it's grown since then given how cosmopolitan Solution Nine is. My only theory for where their surplus of souls comes from is the rest of the shard's people who died during the Storm Surge got absorbed by Everkeep, which is actually kind of horrific, but doesn't seem right because according to Emet-Selch, a Calamity causes the extant shard to be combined with the Source, souls and all. The Source's souls are more powerful because they've already absorbed those of the shards seven times over.
Let's say they do have enough souls to grow, though; do parents need to buy souls when they're pregnant or conceiving? That could've been an interesting detail that added some dystopian flavour, but there's no indication of it. Instead we're told children sometimes become catatonic because... too much lightning, and it has nothing to do with the plot.
When I played through this segment, I initially assumed that the reason the Soul Regulator system is so ominously foreshadowed was to set up this very twist; the Alexandrians were actively harvesting souls from the people of Shaaloani because they need more to keep growing and stockpiling them. If souls are bartered and sold, then over time perhaps they'd concentrate in the hands of older, wealthier residents who want security, depriving young people and those who couldn't afford them from starting families (absurdist science fiction scenario, obviously). But no, it turns out Sphene ended up harvesting souls for a different and somehow infinitely more retarded reason, and all the buildup to the Regulator system was just pointless misdirection.
Assume Alexandria does have a surplus of souls to maintain and grow their current population even as they buy and store them for personal use. Now that they're a part of the Source, any children would have souls reincarnated from the Aetherial Sea; say a generation passes, and those children die of natural causes and get absorbed. Aren't their souls being taken from our natural system and fed into Origenics into a closed loop? That might not sound like a huge issue but the population density of Everkeep looks insanely high compared to any of the less advanced population centres of the Source. I'm no expert but just judging by the skyscrapers and apartment buildings, I'd believe Solution Nine could have a populace in the hundreds of thousands if not millions. Sure, right now it's just a weird cultural quirk but couldn't their habit of absorbing and storing extra souls have pretty big implications for us over time if left unchecked?
I realise I'm massively overthinking this and I'm probably missing something crucial about the reincarnation system. But this was where my mind naturally went while playing through the MSQ and the questions it raises are a lot more interesting than the plot we actually ended up with.
