The thing many seem to forget is while many Special Forces use M-4 (or these days more likely to be HK416), they also issue significant numbers of 7.62x51mm weapons as well. In fact while the US Army is / has replaced the M-16 with the M-4 in the majority of their infantry units they have also increased the number of Designated Marksman Rifles to two per squad from the previous one or in some cases zero.
I imagine with the success of the XM-25 grenade launcher trials and its progression towards production, combined with the increased issue of DMRs and the development of improved SAW / LMG options (i.e. cased telescoped ammunition etc. or more conventionally, lightweight 7.62x51mm options) 5.56x45mm will progressively be relegated to use in carbines carried only by vehicle and support weapon crews, scouts, squad leaders, officers etc, i.e. in the roles previously filled by SMGs. It is probably only a mater of time before militaries begin to discard there remaining 5.56 carbines in favour of PDWs that have been designed specifically to cover these roles and are even lighter, shorter, easier to use and manoeuvre and ballistically more effective out to 300 meters than the M-4 etc.
