And The M7 Is Already Being Modified

 It's only been a few years since the M7 was officially adopted by the Army and we are already seeing some significant changes to deal with shortcomings that were already obvious when it was first adopted such as the weight and recoil. From "The Army’s New M7A1 Spear: Shorter, Lighter, and Finally Making Sense?" at The Truth About Guns. Some of the changes:

• The barrel has been shortened from 13 inches to 11 inches in order to save weight. Now remember that the whole point of the 6.8×51 cartridge and a new rifle was to provide better penetration against body armor (which is primarily an issue of velocity) and longer effective range, so cutting two inches off the already short barrel length just degrades the cartridge's performance. The consequence is that muzzle velocity has now dropped to under 3,000 fps, to now about 2,940 fps.
• The barrel profile has been changed to also save weight.
• Improvements to the handguard to add more attachment points, allow better access to the gas adjustment, and strengthen the attachment to the receiver.
•  These changes and the shift in the point of balance toward the rear makes the rifle quicker to the shoulder and easier to use.
• The folding stock has been abandoned.

On the other hand, there is no getting around the fact that the ammo is heavier and bulkier:

     A key point from the Rangers who have worked with the Spear in testing the ammo load penalty is real.

•  A squad equipped entirely with 6.8×51 carries roughly 60 percent of the round count they would with 5.56
•  Every magazine is heavier
•  Belt-fed guns and DMRs add even more weight on top of that

That matters for fire superiority. Thirty rounds of low-recoiling 5.56 that troops can shoot quickly and accurately still count for a lot in any fight.

    This is where the role of the M7A1 starts to make more sense. Several experienced voices in the video argue that this should not outright replace the M4. Instead, it makes more sense as a special weapon in the platoon.   

And that special role is essentially that of a designated marksman rifle.