Back on topic.
In my opinion the best fix would come with a new cartridge that would blend the requirements for assault rifle and general purpose macine guns.
a true intermediate round that combines good terminal performance, with solid penetration capabilities to 700-800m. Something a bit larger than AR15 magwell, but smaller than .308.
I think something like the 7x46mm Murray would be a good balance:
The 7.62x39mm Russian case extended to 46mm and necked down to 7mm.
the cartridge pushes a 130gr. bullet, with a bc of .411, at 2650Ft/s out of a 16 inch barrel.
The 7x46mm is designed by the same person that designed the 6.8SPC, but is not limited by the AR15 magwell. This allows the loading of good BC bullets in a case design that helps feeding and extration reliability.
A rifle for this cartridge will be heavier than a 5.56 rifle but on the other hand a belt fed machine gun for this round, is going to be much lighter than the 35lbs M240.
This cartridge could be used in all rifles, carabines, designated marksman rifles and machine guns that have to be humped by the troops, dramaticly increasing the firepower in rifles and carabines, while maintaining almoast all the capabilities of the machine guns.
Every time people looked for the "ideal " universal combat cartridge they came up with something similar, but of course, everytime politics and too powerful generals, stoped them from becoming reality. Outstanding cartridges like .276 Pedersen, .280 british are two excelent examples.