Considering the failures that have become more controversial since the adoption of the M4 variant over the A2, one has to wonder what might have happened if the military had just stuck with the 20″ barrel.

While not as handy, there is a marked difference in effectiveness between the 20″ and the 14.5″.

The other option is an already proven and in production choice, the 6.8SPC.
A bolt head, barrel and magazine change and good to go. Reasonable recoil, better terminal effect and range, and minimal changes to gear, logistics and training.
An armorer level change, it is the logical choice and for that reason alone will never happen.