A lot of people forget about the speed factor. Also that 30 mm cannon is gold on the battlefield. I dont think people really grasp what a 30 mm round can do. Then there is the load out. You are hard pressed to find a plane that can carry as diverse a load out as an A-10.
The only plane that could take its place is the F-15E. However it lacks a good cannon. Also its fragile compared to a A-10.