This is a patrol craft
This thing is designed for patrol use. There are a lot of UAVs in the sky already, and they aren't crashing into mountains very often, or being lost to enemy fire. And the use case, is patrol, not an active war zone. So, it will be searching for hostiles, who are trying to go unnoticed.
Yes, at 3000 metres, some large caliber weapons could hit it. But also, bullet source detectors would instantly pin point the direction of the fire. Enemy forces would be smart enough to not fire at it, because it would reveal their position. They wouldn't be able to destroy a P-791 and leave the area, before it located the source of fire, and transmitted the location, and a Reaper is diverted to the area. And the Reaper would only have to get within 8km before it could start firing back.
Radar cross sections don't matter either. Hostile forces in Afghanistan aren't lugging around radar sets. When they are attacked from the air, it is usually a Hellfire missile from below the horizon, or behind a mountain, directed by a laser designator on a UAV, or a US military ground patrol nearby. I bet the P-971 will be loaded with target designators.
I think there just be might UAV overload at some point. They can UAVs cover every sq km, but who is going to monitor and sign off on targets? Yes, a UAV that can stay in the air for a week, cuts down on ground crew, but you still need a 24x7 team watching the feeds.