I guess I'll stick my toes in the water here. It sounds more like caliber selection than weapon selection on one point, reliability versus accuracy on another, adaptability to the situation on another.
Well here go's. I've owned ar15's and still do, M1a's and still do, M1 garands but don't now, ak47's not now, sks's and still do, HK91-don't now, M1 carbine - not now, enfield smle's mk1 and mk2, o3a3's - still do.
None are select fire.
Reliability for all 100% with decent ammo, not match but good surplus.
Accuracy, ar15 1st by a hair, not including my match gun, issue barrels and under 300 yards.
2nd my M1a service rifle, consistently shoot 89% possible score with czech 147 ball surplus, jumps to 94% with US Lake City match 168grn. Beats the 5.56 hands down beyond 300 yards.
3rd my a303, 2 minute gun, but holds it way out there.
4th M1 garand, 2.5 minute gun and no complaint.
5th Hk91 2.5 minute gun, mainlydue to trigger and less than perfect sight picture or adjustments.
6th Sks, 3 minute gun, but holds it out to 500 meters, 15 inches does not sound great, but from a 200 dollar rifle with a crappy trigger and poor sights using chinese milsurp ball, very usable.
7th Ak47 3.5 to 4 minute gun, I like ak triggers, long and light, better than most issue triggers for close fast work, and not bad for careful application.
I would not feel under gunned with any of the above but concessions have to made about caliber. 5.56 allows us to carry many rounds into the field, but does not have the penetration against things bad guys like to hide behind. ( good guys too ) but on a raw target unprotected, yeah it works just fine. On just about any hit on a human it makes bad juju. Head, torso or extremities.
The 7.62 x 39 round is no joke, I shot a match that the first 5 places were taken with sks's and ak's due to penetration, at the last part of the run, you had 20 steel targets propped up on logs( that you could not see, because the grass was too high )
and with the 5.56 your normal aiming point was into the grass for a center of mass hit, that round would not penetrate the logs to reach the steel. But boy did that 7.62x39.
I'm a big fan of the .308 and 30.06, it will do both jobs, but at a cost of weight, ammo count + weight, I've carried the M1a's and Garands in the field, and you want to sling it as soon as possible, bless you guys who carried them for real, ready for action, it gets old fast. If you and your units bacon is on the line, do what you gotta do. I do certainly like a rifle I count on to 600 meters, to hit hard and accurate. The latest designated marksman m4's do that with the mk77 ammo.
The extractor on an ar15 compared to the rimsize is just as robust as it's counter parts. I too like the gas piston system versus the direct gas pressure on the bolt, but guess what, you can get that if you want, and will probably be the next evolution of the AR platform, that is the coolest thing about AR's, kind of like chevrolets small block engines, what ever you don't like can be remedied. Caliber, trigger pull, sighting systems, stock configurations, options for sighting or lighting, bipods and a host of other options. You can't get that on a Garand or a303. Just now coming out for the M1a, Ak47, and even the Sks. But it can be had.
As to the design, remember Eugen Stoner built the rifle for extruded powder and 500 rpm, it was the dod, who said at the last minute, it needs to use ball powder ( changing the burn time to put more fouling in the gas tube ) and 650 rpm. It was designed wonderfully. Unlike the other guns I have mentioned, it has evolved, only a few of the previously mentioned rifles have to any degree. The HK91/93 had the sniper versions with better sighting systems and trigger. Only recently have the M1a's been paid attention to, I think mainly because the people in the sand box have issues with penetration with the 5.56. I for one am glad to see the m14 come back to life. ( except for the milsurp ammo drying up, but if our guys can use it, go for it and if they run out, I'll send em my stash ).
As to the mass of the fire control, it is dimunitive to match the size and caliber of the rifle. Granted, the 5.56 has plenty of pressure, but all things considered, it does not take a big hammer and spring to set off primers, and the buffer spring is humongus compared to any of the above mentioned firearms. They also do not have a buffer at all.
You posed a lot of scenarios, in my home it's 12 guage and 9mm first to allow me to get to my AR15, if I gotta go play with some notice my M1a will be there, along with the aforementioned. My sks with the 75 round drum, ( works flawlessly) also has it's place. My M40a3 also has it's place as does my 26" match AR built for 80 grn vld Berger bullets. 800 meter mouse gun. I would definitely like to have an AR in 7.62x39 that accepts AK mags to complete the armory.
A firearm choice has always been built on reliability , power, distance to the target, size of the target, how many targets could there be, accuracy requirement, conditions of deployment, ( needs are different in a tank than out of one ) and what are you personally willing to carry.