What they do suggest is that if some cartridge is to replace the 5.56
-- and have as many of the ballistic advantages of the 308 as possible
while still maintaining the dogface hump/logistical advantages of the
AR/M-x platform -- then the Grendel fills that bill.
I would say the title of this specific thread is exactly the evidence that is right under our nose - the 6.5G fans have created a concept that it can replace the .308. That's exactly what the OP is talking about.
As for the Grendel filling the bill to replace the 5.56 and emulate the .308 in the process, it goes right back to a bullet with 20gr less mass and 20gr less powder doing it - which is physically impossible. What the general public keeps focusing on is fps and accuracy, where the military is looking for controllable power. Completely different thing. They don't need to bother with making the 5.56 do more, they already turned down the 6.8 which is 40% more powerful and even paid for it's development. Not what they needed because they have other weapons in inventory, in the field, and much more deadly in delivering power. Most of those are crew served.
That's what's wrong with the myopic perspective of caliber debates, as most of the participants are taking it at face value the single shooter needs to deliver the power. It's a classic misunderstanding generally brought up by those who aren't trained in firepower management on the battlefield. They aren't Infantry, and they sure aren't command level trained.
No clue what it's really about. "Gunner, troops in the open, 450m, fire at my tracers!" and another 100 round belt of .308, .50BMG, or a burst of 40mm goes there.
Don't need no stinkin' leetle 6mm's. It's really a question the Army didn't ask and could frankly care less about.