The A-7E was a superior AC to the A-10 when armed with the
30mm caseless chain gun. To upgrade the A-7 to an AC with the F/A-18
perfomance would have cost appr. 3.5 million per copy.

The A-7 could have perhaps gotten F-18 thrust ... that's different in many
respects from F-18 performance.

The A-10 is nicely optimized for the hostile CAS environment with two
well-separated engines, an armor tub for the pilot, etc. It lacks the range
and speed of the A-7, but that's not the prime driver for the mission. You
could also hang a large gun on the F-15E (arguably the best strike fighter
in the business) and kill tanks. That doesn't make it the best CAS
aircraft.

R / John