Look at trends in both adopted and proposed machineguns. Various nations adopted ARs and LMGs like the Minimi/M249/Mk48. The Russians have a 6mm. The US proposed a 6mm. Various commercial solutions have been presented in 6/6.5mm and 8.59mm. There’s a growing divide between machineguns meant to be hard mounted and those meant to be carried in the squad or platoon. The idea that one gun will do all infantry tasks is dead.
If a machine gun was designed and produced in 8.59mm, or .338 Norma, whichever you like, that would still classify as a medium machine gun/GPMG. Which is in the name: General Dynamics Lightweight
Medium Machine Gun. And it would be used alongside the squad-based weapons as a replacement for the M240, I'm sure, in the same role: For reaching longer and hitting harder against targets that the M249 or whichever over 5.56 LMG can't work as well against without necessitating lugging around an M2.
As for the idea of a single gun doing all infantry tasks, I don't think that's existed for a long time. For a basic soldier, yeah, his assault rifle is going to be like everyone else's assault rifle, but on the squad/platoon level you still have individual weapons that fill needed roles, from shotguns to grenade launchers to machine guns. The US military relearned a long time ago that the M16/M4 wasn't going to cut it in every task, so they reintroduced the M14 as a "battle rifle"; the SCAR-H has received a warm reception from the Special Forces community in that regard as well and other militaries use it and the HK417 for the same reason (i.e., the 5.56 has its limitations that the 7.62 surpasses without being too heavy). And now there's suggestion that they want to switch over their 7.62 guns to 6.5 Creedmoor for the better ballistics without much of a parts change. Because of that, I disagree with the notion that eliminating the GPMG concept would be a good idea or even that it's happening. They have a requirement on the battlefield, whether it's 7.62 NATO, 6.5 Creedmoor, or .338 Norma.
@shoobe01 I more or less use the term GPMG and MMG to mean the same thing in modern terminology, because the distinction is now so very much muddled: 1-2 man per gun, bigger bullet than the 5.56 but not as big as the .50, intermediate range and power, suitable for both infantry and mounted use. From a historical standpoint, the German MG34/42 was closer to the "General Purpose Machine Gun" concept vs the "Medium Machine Gun" concept exemplified by the US' M1919s: It was man-portable and single-man-fireable, could feed from either a unlimited belt or a shortened one kept in a drum fitted to the weapon for squad use, but it also worked well as a vehicle-mounted weapon. The biggest difference was that back then, you only had two options for a cartridge: Normal-big (standard rifle cartridge) and extra-big (.50-cal). Maybe things were a bit better back then...
But as I said, I don't think we're going to see a decline in use of guns like the M240, Mk48, PKP Pecheneg, or whatever others are out there any time soon. There's no reason to get rid of them, they still serve a requirement, and I reckon that sooner or later someone would decide to reintroduce a GPMG/MMG into the mix the way they did with the M14/SCAR-H/HK417 long after it was decided (perhaps somewhat foolishly) that the 5.56 would be the default infantry cartridge. After all, the Russians decided that the 5.45 would be
their new default but they certainly never stopped using the 7.62x39 and are still bringing out the newest AK variants in both calibres because they still fill a required role.