Is the Dark Forest theory actually valid in our real universe?

After reading Three-Body, I keep thinking about this. The Dark Forest theory is based on two axioms: 1) Survival is the primary need of civilization 2) Civilization continuously expands, but matter in the universe is constant.

But in our real universe, interstellar travel is incredibly slow. Even with curvature propulsion, contact between civilizations would be extremely costly. If two civilizations meet, mutual destruction might be more expensive than peaceful communication. And if many civilizations exist, surely some would choose not to attack.

What does everyone think? Is Liu Cixin's Dark Forest an inevitable conclusion, or just one terrifying possibility?