Under the Red Queen hypothesis, a prey species that keeps evolving faster defences may end up no safer than it started. Why run at all?

D. Because the predator is improving too, so standing still loses

Both sides improve, so absolute gains cancel and only relative position matters. Stop adapting and you do not hold your ground, you lose it. This is why sexual reproduction is worth its enormous cost: shuffling defences faster than parasites can track them is the whole return.