A video posted on Reddit showcases the sad reality of a Mercy player getting hooked off the map by a Roadhog and not having anyone in their line of sight to fly to. Mercy can drift slowly downwards thanks to her wings, but cannot stop herself from falling off the map. The Overwatch clip showcases this experience from the enemy Roadhog's perspective.
The real struggle of Mercy environmental deaths is the lack of teammates in the area. Mercy can fly to any teammates she can see to get back on the map, but every second she drifts further down and can see less and less of the map.
The easiest way to solve this problem is by using team chat and asking for help, but many players that don't or can't use team chat struggle to draw teammates to them. Spamming "group up" and other communications is a good idea, but is unlikely to actually work. Unless a teammate is paying attention and sees the Mercy get hooked off the map, Mercy will usually slowly drift to her own death.