I will also politely disagree with RDAINES (because this is THR,if we were on AR15.com I would cry and moan and call him names J/K)
I have several high round count ARs and I have had to replace only a couple of small parts. I can't speak about combat-I haven't been there But it seems to me a man with an AR,a basic cleaning kit and a couple of odd parts,say an extractor and a couple of sets of gas rings is going to shoot a small mountain of ammo before he would need to go looking for a new gun.
I have become the guy that folks at my gunclub go to for info and repairs on ARs and about 97% of the time it is because they took it apart when it didn't need it and lost something or because they have found some kind of junk reload ammo that is not as good as wolf or because they didn't bother to put a couple drops of oil on it after they cleaned it last.
About a year back someone at my club sent me out to the range because another member couldn't get his AR to work.
This guy had a brand new bushmaster that he was badmouthing because it wouldn't cycle . It was a patrolmans carbine with a high dollar scope on it. (close to $2000) He had a big rubbermaid tub with 50+ 30 rd mags all loaded. He read the owners as for as the page that told him to clean the gun before he shot it but he hadn't got around to buying a $3 bottle of CLP. He Knew that CLP was recomended so he was afraid to use any other brand of oil and was shooting it dry . He was about 300 rounds into it when it started to not cycle.
I ran a chamber brush in and out a couple of times and put a couple of squirts of CLP on the bolt,lugs,and trigger and handed it back to him.