Here's a Saturday afternoon Rant for ya....
The M14 didn't need replacing. What happened is that there was a generational shift in our soldiers. In WWII/Korea the "parents" of the baby boomers had come from the depression era. The culture of most people of that generation was totally different. Conserving ammunition was something ingrained in marksmanship; ammunition was expensive, and you didn't waste a bullet on something you couldn't kill. A lot of people grew up in rural, undeveloped areas... because most areas WERE IN FACT, rural and undeveloped in this country. Thus most people were used to shooting at we would consider long ranges to kill game, or defend their land, their livestock, or whatever necessitated the use of a rifle. As a result, .30 caliber and larger repeating rifles were mainstream due to their utility and practicality. In other words, THEY WORKED IN THE REAL WORLD, and as a result, that is what drove the products brought to market. People were also better shots, better marksmen, as a result of shooting at longer ranges which took more skill to master.
When the baby boomers went to fight in Vietnam, a paradyme shift in culture and progressivism was in place. A good amount of soldiers didn't come from shanty farm houses, they came from urban and suburban neighborhoods, with two cars in the garaged and a TV antenna on the roof. The discipline of marksmanship was in far less quantity than it had been 40 years prior. Technology was also pressuring "guns" to change. Social changes in American culture were also on the rise, and plenty of youth fell in love with the idea of counter-culture rather than embracing the ideas and skills of their ancestors.
The result is that they wanted to find a way to stuff MORE ROUNDS on a soldier. They had to go down to a smaller cartridge in order to get it done. The Armalite rifle looked cool and futuristic, and they wound up making the military buy off on it. Now these military brass and political acquisitions trolls didn't buy-off on a .22 caliber lightweight rifle because it had been tested through decades of warfare, or practical use by farmers, settlers, and law enforcement officers... NO, they bought off on this entirely new concept because it simply looked good on paper. It was all pure theory.
The M14, a vastly superior weapon in almost every aspect, fell victim to a generation of Americans who could no longer shoot, was replaced the poorest direct-impingement battle rifle design ever fathomed short of the French sho-sho. The newly christened M-16 rifle was utter garbage. It quickly got a reputation for malfunctioning under field conditions. The lightweight bullets were very unstable when shooting through foliage and cover and rarely held their intended trajectories. Effective range had been reduced to crap. Armor penetration was neutered. It couldn't out-range the modest AK47 rounds, and accuracy was only marginally better. The lack of a piston, "space age" stock and fore-grip material, and aluminum receiver made it extremely lightweight, again, in keeping with the "less weight, more bullets philosophy"; however, the gun's underperforming in every other aspect just couldn't make up for the gap.
Today we are still suffering from the same insanity-flavored Kool-aid that the baby boomers drank. We have a generation of kids growing up playing video games that think dumping mags and large volumes of suppressive fire is the only way to win a battle. Nobody ever gives a fleeting though, except maybe the Marines, of building a corps of TRUE marksmen, actually hitting what you are aiming at, at a range the enemy cannot match. The M16 being the one-size-shoe that government has chosen to fit all of our soldiers, has filtered back into the civilian world and created a Billion dollar industry around one of the worst forward-assist-wearing, small-caliber carbines ever to ride out the ass end of a CNC machine. Every new company that comes along trying to make a buck has "fixed" the AR15; that their ultra tactical-looking, "operator approved", LASER-forged version of the EXACT SAME RIFLE is supposed to **** magical bullets, for a mere 10x the cost of the other guys. Its nothing but a massive marketing ploy built around a **** gun. And unfortunately the M16 and all of its variants are the STATUS QUO, dug-in like an Alabama tick until something comes along and dislodges it. I mean seriously, how many of these phony "government weapons trials" do we have to pretend are happening every year before we actually replace the M4? I know the acquisitions world, and its not set up AT ALL to replace the M16. These idiot commands are run by 2 and 3-star generals, who have a budget, who put on these dog and pony shows for new weapons every year. They generate a report that comes out like 4 years AFTER the acquisitions trials, gets lost in Congress and is DOA when it hits the desk of SECDEF sometime in the next decade. Its a show... 100% a show. If you ever showed the OSD comptroller the bill for how much it would cost to replace every M4 in the US inventory with a SCAR16, or something even better, they'd look like Daffy Duck running through the Marsh on the way to the Capitol Building. Its just NOT going to happen until we get outgunned by the Chinese.
Back to the evolution of the AR15....
Now I will grant you that by adding all of these (modern upgraded) components which were never supposed to go on these types of rifles in the first place, match barrels, competition-style accessories, etc, the weapon has been substantially upgraded, but its like putting a corvette engine in a 1978 Pinto... underneath all the added speed and handling, you're still driving a Pinto... so whats the damn point of the vette engine?
So yes.. the M14 didn't need replacing, it was the soldier that changed.