I, too, feel that since the 762x51 has been adopted by NATO and is a proven round that it will be the caliber of choice. How many stories have been repeated here and elsewhere about squads being outfitted with an old M14 to a SDM? No one says that the .223 isn't powerful or accurate but sometimes it's too much of a good thing.
I bow to anyone here with military experience and their opinions as I have ZERO military experience. However I do have a LOT of experience handling/owning MBRs from around the world and one of every semi-auto made. After dealing with about 25 calibers between rifle and pistols, the 725x51 became my go-to and every rifle I've fired in that caliber impressed me. At 100 yards, we set up a solid wood duck decoy. I shot it 10 times with my 55 grain AR, hitting it all ten times while it sat on a stool. It didn't move and, frankly, I thought I had missed it entirely but when we examined it, there were ten perfectly drilled holes though that Maple duck decoy. Then we setup for 762x51. It was shot once and that was it as it just blew up. Both rounds were FMJ and military. I've had the same experience when shooting competition with bowling pins. The other competitors were shooting their really excellent ARs. I brought a French MAS 49/56 in 762x51 NATO with a 10 round clothespin mag and I had no problem dropping pins while the AR fellas were constantly shooting through the pins.
Just to put a wrinkle in this discussion, if I were to select a second-best caliber, I'd probably go to the AG42b caliber of 6.5mm Swede. That ammo hunts really well and is, well, beautiful in it's own right. For a while it was out there in surplus but has dried up but it would be a good compromise, IMHO. Trouble is, however, the rest of the world is NATO and as it's been said, it would be impossible to replace the literally billions of rounds already out there with something new. My favorite rifle today? Also my DPMS LR308B 18" barrel with Leatherwood "Camputer" scope that will keep it accurate to 800+ yards. I've only used it to 300 yards, however, but it was incredibly accurate at that range. I don't doubt it would do more as proven by so many other shooters of this machine.