AC-130 is significantly more of a sitting duck to infantry-deployable surface to air missiles. They have to be fairly low and fly in slow circles with the broadside facing the target. It would be hard to imagine a better missile target. One was shot down in the gulf war by one such infantryman/SAM and all 14 crew were killed. A-10s can loiter and provide extended ground support, but can come in/leave quickly, maneuver well, use terrain to hide from ground fire, etc.
I agree with you, but the original description was that of a cargo plane turned into an assault platform. I was assuming the intention was creating something more along the lines of the Flying Fortress, but the AC-130 was the closest thing I could think of that's still in service.
