I was doing "tabletop" studies of the requirements for a 5.56 replacement in the early 1980's.
And here in 2017 we're still using 5.56
The Army will eventually replace the 5.56, but just as the 5.56 was a modification of an existing cartridge, not an adoption of an existing cartridge, we can expect to see the new cartridge be something specific to the military's needs, not adoption of something that already exists. And given what has to go into such a switch, I suspect only the youngest readers of this site will live to see it in widespread service.
For most of us, the service rifle round will be the 5.56 for the rest of our lives.