But why would dispatchers be standing there for a through service? There are times where a platform is extremely busy and the only place for a dispatcher to stand, be visible, and control the platform edge is being past the yellow line.
A driver would likely blow their horn on seeing someone close to the edge, regardless of a hi-viz. Station staff would raise an arm, just the same as NR track workers would to acknowledge the horn sounding and, if they had concerns over the distance, would retreat away temporarily.
It may seem strange to you that they do this, but they have backs towards the train so that they can see as much of the platform as possible whilst the train is departing. Stopping, or drawing attention, to anyone approaching the train.
The train itself is the responsibility of the train crew first and foremost, be the DOO or otherwise. It's why the rule book specifies that a guard must be at a window, observing the train fully off of the platform.
How likely is a trip or fall likely to be? Can't say I've ever seen, or been made aware of a member of staff falling trackside or down the side of a train during dispatch. That has to be such minute odds.