We are actually investigating a system that allows to have "lightweight" NPC objects that can act as inactive data stores in the background, won't physically move around or interact with the world and have much less overall properties stored in memory.
Such a system would allow to retire a part of the population into that data structure and also turn lightweight NPCs into full-fledged ones. For example retiring the elderly part of the population or graduating students to the background and at the same time have younger students as background characters who can become full-fledged NPCs that can be used in events once they pass the minimum age threshold. (They won't be part of normal NPC lists or appear for selection in the game until they become full-fledged objects, so no need to worry about suddenly having characters below the minimum age in existing events.)
The system is somewhat intended to make it possible to close the age gaps that we have in the population, where you basically have students who are 18 and their parents who are 15+ years apart and pretty much nothing in between, due to how parents and students are always created as a family and offset to plausible ages. And if you wanted to include even older generations of family members to connect different branches of a family, you'd need to add more elderly people which in turn could skew the overall population composition away from the group that is the focus of the scenario.
However, all that would still not solve the problem of having your "corruption progress" being reset constantly by having the corrupted students graduate and uncorrupted students coming in, unless we'd adjust their starting stats based on the overall average of your school. But you'd also lose your interpersonal relationship progress with the students who leave. And for story reasons, you'd not want to have quest-relevant students graduate either. So it's questionable whether such a graduation system would be desirable at all.
Side note: An "Incubus City"-like personal goal is actually planned at some point. But it obviously does not make sense to introduce such a thing while pregnancy is still not part of the base game.
