Smart AIs have a shelf life of seven years before they start to malfunction and undergo rampancy. However, Cortana states that entering the domain cured her rampancy, since she now has infinite memory storage. However, you can still clearly tell that something is off with Her.

Would you prefer they put in a time limit before something happens with the Guardian? Because people would most likely complain about that. Besides, there are plenty of times in the old games where you were supposed to be running like hell, but could still sit motionless for as long as you want.

Except that’s been part of Halo lore for a long time, and Chief had to be told it because he either never thought about it, wasn’t taught about it, or didn’t know when Cortana came into service.