What are the effects of the college deploying massive efforts towards capturing apparitions? Is the population limited in the physical space?
(If it is, I can see it miscast risks of apparitions for apprentices in the empire lowering just so slightly).
You'd need a lot more solid answers about the exact forces that give rise to them to say for sure. You could posit that there's a set amount of them or singular creation events that give rise to a particular type and preying on them will reduce their number in the long term. You could also posit that removing them will just create a vacuum that will be swiftly filled.