The M-16/AR platform is not limited to .223/5.56. An AR in .308 is lighter, more accurate, and more dependable than a M1.
The military could have easily chosen the Stoner AR10, .308 cal, however, they did not. They needed a longer range semi-auto rifle, the M14 was still in use in some units, so it was chosen for the upgrades.
The article goes over the upgrades and performance of this rifle, not the pros and cons of what rifle makes the most sense and what would have been the best choice. The comments quickly became sidetracked by opinions on what rifle and caliber is best, that was not the point of the article for those that bothered to read it.
Reading is fundamental.