The P-51 sucked at ground attack against even post-war systems, as we found out in Korea. You do not send a water-cooled piston engine fighter up against automatic weapons fire - a lot of the P-51 losses in Korea were due to ground fire taking out the radiators and therefore the engine. This is why the A-1 Skyraider took over the role and was so successful.
Another case for “civilian airliners that look exactly like a military aircraft end up getting shot out of the sky”: https://en.wikipedia.org/wiki/1954_Cathay_Pacific_Douglas_DC-4_shootdown
A-1 … a whole new concept! Yes, the P-51 was designed as a long-range escort with the capability of acting as a fighter-bomber. In the latter capacity, it attacked German planes on the ground or during take-off or landing, as well as other targets of opportunity. This fighter-bomber capability was very important for destroying German jet planes (and their pilots) (think of the ME-262) that we could not fight well in the sky. The concept of the A-1 (embodied in the A-10 of today) is a plane that can survive while operating in a ground attack role when the enemy is throwing up a lot of small arms fire. Because the vital organs of the plane are protected by positioning and armor plating, the A series plane will often come back from a mission full of holes that would have crippled a plane not designed to operate in such an environment. I suppose there is a trade-off of speed, range and maneuver to gain this level of protection. I did not know about the A-1, so thanks for the tip.