Derry of course needs an airport for economic development. It would be economic suicide to close it.
Thinking in NI terms is absolutely ludicrous. There is no border anymore in practical terms.
Dublin is Belfast airports biggest competitor not Derry and as someone who lives in Belfast, the 3 airports I use frequently are Dublin and both Belfast airports. If I could hop on the train that took me directly to Derry airport, I would use that too.