This seems like part GQ, part IMHO… so if there’s some opinion in my post, that’s why.
The AR platform shits where it eats. It literally fires burnt carbon and particulate matter into the action in order to cycle the bolt, which I think is a massive disadvantage. This is negated by an extensive infrastructural support for cleaning and maintaining the sidearms, but it’s something that has to be factored in, either for a personal purchase or for a military one.
I wasn’t trained to use the AR platform in combat, so my reload has always been Drop Mag (Right hand) while (left hand) puts empty away and grabs new mag, left hand goes back to position and on the way, flips the bolt release. I’ve never needed to use the forward assist mechanism, because I’ve always been meticulous with the ARs I’ve fired.
On the other hand, a gun like the F2000 is either "Grab magazine with lefthand, put away, put in new magazine " or “Left, on the way back, locks bolt, grabs magazine, puts new magazine in, releases bolt on the way back to position” (you can see the charging handle, here, on the lefthand side, towards the front of the firearm).
The F2000 also uses a piston system, like a lot of other evil black rifles, which doesn’t dump burnt material into the action of the firearm. It also has a “Fouled” or “low-charge” setting, where the gun will still feed if it’s heavily fouled or the rounds are undercharged.
I’m using the FS2000 here, because I own one and I know the gun well enough to speak about it where I don’t know the AUG or H&K action designs, although my understanding is they’re similar. I also have a pretty good understanding of the SCAR-L/SCAR-H, which have superior actions to the AR (in that they’re piston systems).
The real fact of the matter is that the AR is a good design, and the US military is heavily invested in it, so they’re not likely to switch (en masse) to any other firearm unless it offers significant, measurable advantages to the AR platform. Was direct impingement worth it? In itself, probably not. But, Stoner designed a good gun, and despite the drawback of DI, the gun itself was still worth it – massively more so at the time.
Again, IMHO.