That may be, but your ability to engage targets at range and during periods of limited visibility is inferior to a rifleman with proper optics.
A good shooter with optics will engage targets faster, further out and with greater accuracy than a good shooter without optics. Nothing wrong with irons, but durable, reliable optics provide a significant advantage, especially when combined with thermal or IR imaging capability for night/all weather target acquisition.
5.56mm is the caliber of choice for the time being - bullet weights may change, the 77gr SMK being the current de riguer fad, but for the majority of rifleman the 62gr M855 is perfectly adequate for its intended use.
It's easy to nitpick 5.56mm in isolation, but EVERY western army has adopted that caliber, NATO and Non-NATO alike - it's superiority over 30cal battle rifles is proven, specifically, the average rifleman gets better hits, faster, with 5.56mm than 7.62mm.
Only hits count. Training helps, but there are limits to how much time soldiers can spend on the range - we can't train every infantryman to handle battle rifles effectively.
And for those of you out there thinking you can run your 308 FAL, HK or M1A with an AR, good luck.
In combat, speed counts as much as accuracy, and shooters of equal ability will get better hits, faster, with a 556 than with 762. Always, always, always.
Infantry combat is NOT about patient long range rifle fire - it's about getting there first, with the most, and lightweight 5.56mm rifles like the M16 series are superior tools for that task.