They should still be using the M60; but in house and cave clearing ops, all that is needed is the AA-12
The M60 is a mediocre weapon. Its modern replacement, the 240, is based on the German MG42 from WWII, and is vastly superior.
Would have made more sense to me to introduce an improved SAW, or just equip everyone with the new weapon. What about this weapon makes it only suitable for one Marine per squad?
There should be 3 per squad.
Remember the difference in use between a machinegun and a rifle. The machinegun is valuable as direct fire, but its biggest contribution is to be able to suppress the enemy so the rest of the unit can manuever.
The riflemen are there to provide aimed fire. As they kept saying in TBS, "There is no burst fire in the Marine Corps!" If everyone is spraying lead, you're burning a lot of ammo, probably with not many hits.