Wasn't the M14 meant to replace 2 weapons ?
Three, actually: M1 Garand,BAR, and M2 Carbine. The BAR replacement never quite worked out, begetting the M15, which was cancelled the day it was adopted.
The 6.8 is an answer to a question no one ever asked.
As a battle rifle, yes.
Army has identified a need for
support weapons at the
Squad level to have greater range & impact than the Squad's combat rifles.
They also noted that the dude humping the SAW never drew off the Squad's ammo loads (other than spare belts draped over Squad members). And, the need for specialized SAW ammo supply obviated the argument about adding an additional ammo supply at the Squad level. As far as the Puzzle Palace is concerned, they are already moving SAW ammunition by the ton for Regiment/Brigade Supply already.
The argument about side arms will never be resolved; neither will the need/best way to arm support troopies.
Most troopies are limited to, maybe, 50m with a pistol. Doctrine suggests that 'bad guys' are meant to be dealt with out around 100m by direct rifle fires, and ranging out to 300m. But, that's "open field" engagements
Now, with many current engagements being MOUT, combat ranges are much reduced--which creates an argument for fielding a PDW in a light rifle caliber.
MOUT engagements also put "support troops" in the thick of it (or fully in the rear with the gear). In that latter case they are best equipped with a USG Skillcraft, if only to reduce ND.
"Support troops" is a loaded phrase, too. An arty crew only 1 km from Forward Edge of Battle is way too close. Truck drivers working with the trains are likely to wind up in immediate (and dire) contact with little or no warning (which is redoubled in MOUT). The Supply REMFs 25-50 KM back? Well, back to that Skillcraft. A mortar crew part of the Weapons platoon of a rifle company? That's a bit more blurry. They are like to be in rifle shot distance (if badly placed) ut working mortars with slung rifles is not a "best practice."