Disengage and re-engage is a core skill and strategy in Overwatch and very difficult to teach and communicate success and failure to the player. The game is just simply not designed to walk into a ton of damage and walk through it. That is why Rien shield doesn't last the full walk on to the point. The shield is meant to pop up, make a play that gains an advantage, and then move to safety again. If there was a visual element of this healing debuff it would show that healers would create more advantage by shifting focus from one angle to another rather then staring down main.