Much of the complexity is in interop. Passing data between ATC control positions, between different facilities, and between different countries. Then every airl

Much of the complexity is in interop. Passing data between ATC control positions, between different facilities, and between different countries. Then every airline has a bidirectional data feed, plus all the independent GA flights (either via flight service or via third-party apps). Plus additional systems for weather, traffic management, radar, etc. Plus everything happening on the defense side. All using communication links and protocols that have evolved organically since the 1950s, need global consensus (with hundreds of different countries' implementations), and which need to never fail.