> technology hasn't increased the number of planes or amount of airspace one ATC can manage

As I understand, the primary limiting factor for airport runway throughput (arrivals and departures) is wake turbulence from the engines. I remember, as a kid in the 80s/90s, that there were some accidents related to smaller planes taking off into the wake of larger planes. I am pretty sure that regular passenger jets (say, A320/B737+) are limited to one takeoff every 2 mins from a runway. (Or it might be 1 min.) That said, improving ATC technology might help to reduce delays and maximise runway throughput.

Loosely related: I cannot remember the website now, but someone posted here in the last 3 years an insane website that showed (visually!) the new approaches to London Heathrow Airport (world's busiest two runway airport). It was batshit crazy. I am sure they spent months designing the new approaches. It looked like multiple DNA helix'es where planes circle to wait for landing slots.