The US was quite different in their requirements. And the AK-type rifle would not meet the requirements at all. Longer range precision accuracy was a requirement, as well as, many human engineering factors (easy to use, controllable, modular, etc…..). The AK really only has evolved with the change in caliber to 5.45x39mm but core design has stayed the same. Brutally reliable, poor human engineering, poor accuracy beyond 150 yards compared to its American counterpart, but it fit the need of the Soviet conscript Army.
Heckler & Koch would launch a very aggressive marketing campaign claiming their HK 416 was superior. The rifle got very mixed reviews. It got feathers riled enough that Senator Coburn demanded the Army do a full open competition test to see if our military had the best rifle. The Army stated that they were pleased with the rifle and saw no need to replace it but they were forced to run the competition. Many companies submitted. It was down to four different rifles. All rifles were tested with the new M855A1 ammunition, which was higher pressure. None of these companies were provided this ammunition to design their firearms around.
All 4 failed the durability testing with the new ammo. The competition was cancelled. The M4 would remain. However, this did lead to an M4 upgrade program. This would replace the barrel with the new SOCOM heavy barrel; replace the burst fire control with an auto; and, make the rifle ambidextrous with an ambi safety and magazine release. This too was cancelled after a brief time. As of 2017, all rifles procured are in the SOCOM M4A1 configuration.
I believe we have reached the peak with the use of the metallic cartridge. Until we see some new form of ammunition (caseless, for example, or a laser gun!) I do not see any change. With the new technology for ammunition, we will see the next evolution in small arms. The M4/M16 has changed with new battlefield requirements successfully since 1965. Through changes in materials, changes in battlefield environment, as well as, all the new optic technology. The ability to change out with different cartridges has yet to be realized within the military (300 Blackout, 6.8mmSPC for instance).
The platform still has much life in it. I believe it will continue to serve until we reach that new thing. I believe the same for the AK47 on the Russian side. We have seen the evolution to the AK74. We have even seen the AN94 officially replace it, yet never go into production for anyone out of SPETZNAZ. It was too expensive and complicated to produce so we have the AK74M. New rifles are showing some presence but the core is still the AK-series. Like everything in life, it is up to speculation and debate, but this is my opinion.