The M-16 and the AR will probably end up being like America's Brown Bess and for the same reasons. History has a way of repeating itself.
British Land Pattern Musket (Brown Bess) - 1722 to 1838 (*Click*)
Think about how many successive familial and social generations 116 years are, especially at that point in history due to an individual's average lifespan. 4 generations? 5? That rifle's time in service was forever. Great grandsons and great, great grandsons using the same model (with some improvements - flintlock to percussion cap) that their great grandfather and great, great grandfather used. The M-16 is at 53 years and climbing. Soldiers now using the same improved design that their grandfathers used.
A British soldier could have seen action in the Anglo-Spanish War of 1727–1729 and then (he himself or) his son or nephew could have been in battle with the same rifle or one exactly like it in the Jacobite rising of 1745. 93 years after its adoption a male member of the same family could've carried a Brown Bess at the Battle of Waterloo in 1815. When it wasn't being used in battle it was often used to protect the home or out getting game. Starting to see the same thing with the M-16. Grandfathers in Vietnam, sons in Panama or the Persian Gulf and grandsons in Afghanistan and Iraq. When they get home it's pretty common to get a semi version of what they had.
To some degree from a technology standpoint the M-16 and its variants and the civilian AR kind of mirror the Brown Bess and its place in the military and society. The Bess wasn't replaced for so long for a few different reasons. There were huge numbers of them. The troops and some of the populace were intimately familiar with them since they'd been used for so long. And as far as the technology went there really wasn't anything better on the horizon. If it was better it wasn't by much and usually presented some drawbacks. Sound familiar?
Like the M-16 there were continual improvements to the Brown Bess as needed. Iron ramrod instead of a wooden one, shorter barrels (less weight to carry and which didn't detract from accuracy), notched rear sight (previous models had no rear sight at all), spring catch to prevent the bayonet from slipping down the barrel, a different shape trigger guard (ease of manufacture) and eventually a percussion lock for more reliable ignition (especially when wet). The M-16 has had upgrades throughout the same way. Chrome lining, faster twist rates, forward assist and so on.
What finally led to the adoption of something else instead of remaining with the British Land Pattern Musket was actually the invention of a new projectile that solved the fouling problem associated with rifles, the Minie Ball.
Currently since the new rifle designs that are coming up don't really provide anything that the M-16/M-4 can't do there's not much reason to go through the astronomical expense, hassle and training that adopting a new arm would entail.
Like the Brown Bess the M-16 has been around for awhile (53 years and a few generations) and if any new arm is eventually adopted by the US Armed Forces I imagine that it would probably be for the same reasons, the new rifle significantly outperforms the M-16/M-4.
Someone else mentioned guided mini-missles and electronic guided optics.
Think something similar to the Tracking-Point integrated guided rifle optic, but an optic that's less vulnerable to being hacked and that works with guided munitions.
From a civilian perspective the munitions wouldn't likely be legal to own by the average individual. The optics are cost prohibitive for almost everyone but the super rich.
It took a long time for the AR-15 to gain the acceptance it has and that's only really happened since 2004 and the end of the '
Assault Weapons Ban'.
Even as recently as the 70's, 80's and 90's if the occasional shooter had a semi-auto rifle it was more likely to be one of an array of different rifles like a Mini-14, an M1 Carbine, an M1 Garand or a Rem 7400. I'd venture to say that most shooters generally didn't have mag-fed semi auto rifles. Wasn't really a thing. 3-gun competition wasn't around and the only sort of competition that was around back then were service rifle competition.
At any rate in my mind there are some parallels between the Brown Bess and the M-16/AR-15. Sure, there will be alternatives to the AR in the civilian world as there are always those who want to be different even if there isn't much real advantage. Not any real competition though. For the Military the M-16/M-4 isn't getting replaced until there are actually some real measurable advantages. Right now I'm not seeing any advantage and apparently neither is the military.