Bullets are a lot cheaper than missiles. Predator drones get the job done when you just need a one-and-done missile shot. But a loitering gun in the sky is a fundamentally different weapon.
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I think the issue is that the primary role of the A10, which was to destroy tanks, has basically disappeared. Tank armor has increased to the point where an A10's gun may not be reliable enough.
The A10 exists as a cheap CAS role that it wasn't ever really designed for. A10 has a loiter time of 90 minutes, while AC-130 has a loiter time of 4.5 hours. The air-force would rather be sending in AC 130.
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That being said: if the Army wants the A10, they should get the A10. Air Force's main job is air-superiority, ensuring the skys are clear for other operations.
I love the A-10, but it's effectiveness as CAS for adversaries with even low-level AA is decreased to a point that its less effective than the F-35.