While it makes sense to me that crew should eventually 'age out' of the pool, instead of accumulating endlessly unless lost with ship destruction, I am highly nervous about the mechanism employed to do it. If it turned out that building the size of navy I want to build, featuring the types of ships I want to use, meant that my empire needed sixty extra Academies on every major colony, I would be pissed. In the fiction of my mind I expect certain ratios of installations. Varying wildly from that harms my believability.
But my main concern is with what I consider the worst bug in all of Aurora. Since version 2 or so officer promotion has included the 'up-or-out' philosophy of the modern U.S. military. As a result, literally every single one of my conventional starts has followed roughly the same path; by the time my empire has researched the necessary jump point theory, grav sensors, jump engine techs (& an actual functioning jump engine design) and constructs its first survey ships, 90% of officers in the fleet have been without an assignment for two-to-three tour lengths and are "deemed surplus to requirements and released from the service."
I fear the same thing happening with crews. I want the problem solved before giving C# Aurora the ability to fire any more of my personnel.
This would not happen, the amount of crew from each academy stays the same after the academy reached its limit in graduating crewmen.
Lets say each Academy allow 1000 crew (based on service length)...
When it is built it will add 30 new crew per month until it supplied 1000. after witch it is abstracted that it is able to train as many new crew as are leaving the service after the service time is finished.
If you have a ship that carry 300 crew that is destroyed and they alll die the pool now suddenly is only 700 and you have to wait for 30 month for the pool to recover.
Service length could also be extended during wartime, but the cost of service time should not be a linear cost. So a service time of 12 month might cost you say 0.1 wealth per crew while 24 month service time might cost you 0.3 wealth per crew. Service time could then be increased during wartime for a considerable cost and would also be a way to cover some losses or add to the pool for new ships produced faster than expected without having to add more academies.
If you increase the service length you don't just get more crew though, you just increase the pool limit and gradually add more crew over time until the limit is reached. If you lower the service time though you will pretty much immediately retire allot of crew as their service time is up... or you can do the same procedure and imagine everyone serve their time but are not replaced afterwards.
The number of graduates per month and academy would also depend on their skill level. Low skill give you more graduates per academy per month thus also a higher pool limit, while high skill require more time and effort and give you less per month and thus a smaller total pool size... pretty similar to now.
It would be a really simple but interesting system to work from.
So the fear that crew would disappear would only happen if you loos academies, from say a war. But you would not loose them immediately, they wold then be lost in the same pace you would otherwise have gained them, but you could mos likely offset this by raising the service length and pay more for them for a while from your other academies (unless all the academies are in one place).
If the crew pool start to dip below the totally required crew for your ship then ships would start to get totally untrained crew and the skill level of the crews on all your ships would start to drop considerably until you fix the situation.
It is a really simple and abstract way of dealing with a more realistic crew system that does not result you needing to keep track of crews individually. You just tie the general pool with the ship and their individual training and skill depend on the skill level of new crew.
One interesting thing you also could do is tie it into the deployment time of ships. As if you give ships a very long deployment time you will have to accommodate for the fact that the crew is especially picked and have special needs. Thus you could incur some penalties such as wealth cost of the crew of that ship being much higher than otherwise. This means you are not going to give ships a deployment time higher than regular service length unless it is really necessary.
This also would make saving crew a pretty important thing to do if at all possible, as saving them will make sure they can join a new ship as the pool size don't go down.
You probably also would want to separate crew training from the Academy into its own building, perhaps also give some technology that can increase the efficiency and pool size of these buildings over time.
If you want crew to be an interesting resource something like this could add to the game without needing any micromanagement.