I see a middle ground here...

Stream CCTV footage from every platform to a control room. Have operators observe the arrival of every train, and have a big red emergency stop button for when people fall on the tracks.

In a big network, with say 300 stations with a train every 4 minutes in each direction on average, that works out to ~40 operators required to watch those cameras for 10 seconds during each train arrival.

Not-so-advanced image processing could be used to show only the train arrivals at the busiest platforms or where there was most recent movement on the track, bringing you down to 1 or 2 operators.

Thats a lot cheaper than both platform edge doors, and drivers in every train.