The M14 advocates often see things from a limited perspective, as if only their choice is available to defend at long range. The reality is as suggested above - crew served weapons do that job. The soldier is NOT alone in some spiderhole sniping at the enemy, he is almost always part of an organized unit with other crew served weapons that have the long range task.
His part is to cover what's left that survive artillery fire, mortar fire, .50BMG fire, MK17 40mm grenade launcher fire, and M60 fire. At that point he might start seeing actual enemy trying to work their way to closer cover. It's NOT a grassy mowed field where you can see targets 800m away, and it's NOT always a defended position where you sit idly by waiting for the enemy to attack. More often you go out hunting them on their ground - and hauling heavy guns with limited ammunition for a close quarters fight isn't the answer.
And why would they not see you 800m away, too? Because YOU do everything to stay out of sight - work thru dense cover, stay below the top of the ridgeline, wear camo clothing, shield glass from reflecting sunlight ad infinitum.
Some of the M14 fans are discussing it's use - but not from a combat arms perspective. And just the same as the M16, what is issued and used by 90 percent of the troops is often never used - they are support, not combat, it's a personal defense weapon. Exactly the roots of the M4 and M1 carbine. How they treated the weapon and what they were told isn't "combat" practices - they rarely practice and usually don't get the memo. They aren't in the loop, just pick it up later. Certain tactics and techniques aren't revealed to them, they don't have a need to know. Their primary job was something else - not trigger puller.
That's why 90% of the anecdotal stories about what someone did "fighting" somewhere are like the evening showcase of servicemen on TV. Most were support - but the announcer always describes their service as "fighting" in part of the war. Sorry, most support rarely go thru a magazine a month. Only in recent times with guerilla warfare and insurgency have they come under fire - '80's on - and as an Infantryman I made sure to remind them of it. I was not coming to their rescue.
As for the AR10, is IS becoming a success, belatedly and in a different form. If the AR10 is seen as simply the .308 version of the AR15, the original, then yes, there are many more now in service than in the past. Britain bought over 10,000 for GWOT use and Germany is backfilling their current arms with them. They are the weapon of choice when a .308 based long range weapon for Infantry use is needed - because a guy with a M14 or bolt action is readily seen as a different shooter and therefore draws more fire now.
Bad mojo to show up on the battlefield with a recognizably different long range weapon that identifies you as a more serious threat. Better to just go ahead and hump an M60, you'll need to suppress a lot of fire trying to suppress you first.
Nobody considers the unintended consequences.
No, the M14 was not and never would be the better fighting rifle. The M1 decision makers blew their chance when they dismissed magazine feed on the Garand. And that may have been a bad choice, as the soldier still wouldn't carry as much ammo as he could shoot, and secondly, full auto would have still be off the table and only disarmed him quicker.
It takes downsizing the rifle with smaller rounds to enjoy that simply because as humans we cannot carry everything we need in combat. Nobody is capable of carrying two rifles with two complete basic loads of ammo along with all the other gear that is required in a fighting team. It can't be done - it's a luxury of the sunny Saturday afternoon square range with targets made distinct out to 800m. That isn't combat, and in combat, the .30 cal battle rifles were proven to be less than desireable.
Professionals - combat veterans who shoot move communicate - made those decisions decades past and are now gone. The M16 is on the verge of being a C&R in it's own right, and certainly has been produced in staggering numbers, even compared to the M1 - over 8 million. For all the defensiveness of the M14 admirers the bottom line is that it's skin deep - wood and steel. Doesn't make it any better a gun than the Chauchat before it. Looks don't make the rifle - but they can certainly sell them to those with no knowledge of it's dynamic issues.
Go shoot 500 rounds thru yours this weekend and tell us about it.