The M-60 is considered a General Purpose Machine Gun (GPMG). In WWII/Korea we had light (M191A6), medium (M1919A4) and heavy (M1917A1, and M2) MGs. The GPMG replaced the light and medium. It also did double duty as a section support gun, since there really wasn't anything else out there until the SAW came along. The concept of the GPMG stems from the German MG42 of WWII. We thought it was a good way to go, so we copied a couple different German designs, added some US designs and came out with the M60. The rest of the world settled on the FN/MAG-58 GPMG. Thirty uears later, we finally also settled on the FN/MAG-58 as the M-240. Gee, looks like we could have saved some time there.
When the Army fields new systems, they really don't dick around too much. Usually they go by whole units in a priority order that makes sense for the system. There won't be a mix of M60 and M240B in a unit or even a major command. They do the switch all at once to prevent parts and support headaches. Figure that with the number of M60s produced, the'll be around for a while.
Ross