[longwave]

> I wonder why they can't reject the flight plan for an aircraft that's already in the air?

You need to know everything that may be in the air - if you skip the details of a flight that may be in the air, you risk routing another flight through the same space and the possibility of collision? So if you can't do that safely, the only option is to shut down; existing flights can continue but no new flights can be routed until the anomaly is resolved.