So everyone's favourite vegetable expletive-laden character had a fate presented to her that wasn't expected by anyone in the show.
In fact, it wasn't expected by anyone, arguably.
Can anyone explain how exactly one goes from healthily (relatively) surviving a gunshot wound in the stomach to being in a persistent vegetative state in the span of a couple of hours, considering modern monitoring equipment in a hospital?
The monitoring equipment in the hospital measures things like blood pressure, pulse, oxygenation and the like. What it doesn't monitor is blood clots. Blood clotting is a normal reaction to wounds and is part of the healing process. What happened to Debra is that one of the clots came free and began circulating around her bloodstream. At some point, it clogged one of the arteries that fed her brain causing her to have a massive stroke. This is not uncommon. More than 795,000 people in the U.S. per year have some kind of stroke and it is the leading cause of death for all Americans according to the CDC
Even in this era this kind of surprise death or near death condition comes.
For the point of view of story gunshot wound survival was just to put up the suspense in viewers mind. Its already decided that she is going to die in the end. Even Jennifer Carpenter doesn't want to extend her role for a spin-off or any new season and want peace for Deb's character.
Maybe direct gunshot death appeared very lame to makers of the show, so that they made this melodramatic death scene.