It won't sag like that. Or allow me to rephrase that, it shouldn't sag like that.
The roof is a hell of a lot less structural than people think. Roll-over, accident protection, etc, yeah, it's very important. Additional rigidity, sure. But it's not primary structure.
Remember guys...this isn't a Cherokee unibody. No sag should happen here to what is basically a half-cab Cherokee. I've thought of this in the past, and if done properly, it shouldn't be a real problem.