The longevity of "Ma Deuce" is, indeed, a tribute to both the robustness of its design and the absence of a revolution in the basic technology of small arms between its introduction and the present day. It is also a function of the frequent repurposing of the weapon. Originally designed (towards the end of the First World War) to shoot down tethered balloons, the M2 .50 caliber machine gun has since used as a ground-mounted anti-tank weapon (in the interwar period), as a vehicle-mounted anti-aircraft gun (during the Second World War), as a means of economizing on the main armament of tanks (after the Second World War), and, on a variety of mountings, as a complement to automatic grenade launchers. It has also been used in a wide variety of naval configurations, usually (but not exclusively) on landing craft, motor torpedo boats, and other vessels of relatively small size. In other words, the survival of the M2 is due less to its suitability for any particular job, but to an uncanny ability to switch jobs at the right time.
John Browning actually designed an unnamed .50-caliber version of the M1917 at the end of World War I. The prototype was a failure and was never produced. Along with others he worked on improving the design, but died in 1926.
The improved M1921 entered limited service in 1923, but large-scale production didn't begin till 1929. It was found to have design issues as well, and was soon replaced by the M1921A1. Both guns were later removed from service during World War II.
Between 1931 and 1933, Dr. Samuel Green worked on the design yet again, and the result was the M2. Unlike any previous designs, it could be easily configured into different types, most notably a new, lighter, air-cooled, heavy barrel (HB). By the end of World War II, all M2 guns had been converted to M2HB.
Since then there's been other variations for specific uses, but I will skip to 2010 when the M2A1 entered service to replace the M2HB. Yes, 77 years is still a long time.
Sources:
[Green, Michael (2000). Weapons of Patton's Armies. pp.31-34.](https://books.google.ca /books?id=a8jk6165Q08C&pg=PA31)
General Dynamics press release, "U.S. Army Awards General Dynamics $35 Million to Produce M2A1..."
Follow up question. The German MG42 designed in WW2 is still having guns very closely based off of it being used today (the MG3 by the Bundeswher which is nearly an exact copy and the M60). I understand the MG42 is actually a very simplistic design which makes it reliable but what else about the weapon has made it such a popular SAW?
EDIT: other than suppressive capability it was renowned for. Things like reliability, ease of use, etc