You might be correct about the military adoptin' the 6.8, but it ain't much. Now a round like the .260 could replace both of the current military standards, with a 125 grain bullet at 2800 a similar round danged near won acceptance between WWI an WWII when the Army officially adopted the 7mm and Chief of Staff Douglas MacArthur nixed the change due to the amount of .30-06 ammo in storage. The 6.5 Grendel could well be that cartridge, but Army ordinance ain't known for smart thinkin'.
As for the .243 fadin' away, that's purely wishful thinkin', its been chambered in too many popular rifles and has sold well in all of them. Millions of folks have one and use it for both vermin and deer huntin'. It is adequate at both roles, and the standard owner ain't a rifle looney who cares about somethin' else bein' superior.