You don't always have to copy the solution as long as the problem is solved.You can't use HMG like GPMG for the simple reason of ammo volume.
Sure the gun is smaller and lighter, but the volume and weight of the ammo doesn't change, so you are only carrying about half the amount.
Not to mention the bulk of 12.7mm class weapons. With a GPMG, you can still maneuver similar to a rifleman, but not so much with a HMG.
What will not having an GPMG but only SAW and HMG mean tactically? SAW should cover distances up to 500m, beyond that maybe HMG is better than GPMG. Current wars have often been against unarmored insurgents. If the enemy drives around in MRAP a HMG could be more useful.
Just like some countries have rockets in squads while others have weapon squads in platoons. If I'm not mistaken 5.8mm is designed to out penetrate 7.62x54r out to 800m
This source thinks that 50cal gives best suppression per KG ammo.