The long-range indirect fire from MGs in WW1 was used against area targets - wind drift had a huge effect on the trajectory at such ranges. The emphasis nowadays is more on engaging point targets.
Others on this forum are much better at crunching the numbers than I am, but I suspect that the .338 Norma firing 300 grain bullets (MatchKing in initial testing, but ball, AP and tracer being developed) at 2,650 fps from the 24 inch MG barrel will offer trajectory/wind drift performance at 1500m much closer to the .50 BMG than the 7.62mm ball.
The 7.62mm can no doubt be improved with a better bullet, although cartridge OAL restrictions mean that longer bullets will eat more and more into the case capacity, reducing the propellant load and velocity disproportionately. This problem would be exacerbated by the extra length of the lead-free bullets the army currently favours.