There are some old fudder dudders who firmly believe that 5.56x45, .223, and similar and related calibers are a "fad" or a "failed experiment", especially for military use. Usually, they advocate a return to the full-power .308/7.62mm caliber as the end-all caliber, but some advocate for weaker calibers, like 6.8 SPC or 7.62x39. To all those who object, caliber is an important design consideration.
This post will be short: 5.56 and other small-caliber, high-velocity (SCHV) cartridges are not a fad. In fact, guns have been getting smaller and smaller in caliber, and higher and higher in velocity, for hundreds of years. Let's take a look at how infantry small arms have progressed in the 800 years or so that they've been around. (The historical overview section of this article is only intended as a cursory overview of the weapons in question. Details may have been omitted, glossed over, or compressed. I highly recommend seeking out further literature on these topics.)
1994 M4 Rifle, 62 gr .224" cal spitzer boat tail at approx 3,000 ft/s
1967 M16A1 Rifle, 55 gr .224" cal spitzer boat tail at approx 3,270 ft/s
1958 M14 Rifle, 147 gr .308" cal spitzer boat tail at approx 2,750 ft/s
1903 (1906) Springfield, 172 gr .308" cal spitzer boat tail at approx 2,640 ft/s
1903 Springfield, 220 gr .308" cal round nose at approx 2,300 ft/s
1892 Krag-Jørgensen Springfield, 220 gr .308" cal round nose at approx 2,000 ft/s
1873 Springfield Rifle-Musket, 405 gr .45" cal lead round nose at approx 1,350 ft/s
1868 Springfield Rifle-Musket, 450 gr .50" cal lead round nose at approx 1,250 ft/s
1853 Enfield Rifle-Musket, 530 gr .577" cal Minié ball at approx 900 ft/s
1700s "Kentucky" Long Rifle, ~140 gr .45" cal round ball at approx 1,500-2,000 ft/s
1717 Charleville Musket, 494 gr .69" cal round ball at approx 900 ft/s
16th Century Arquebus, 464 gr ~.68" cal round ball at approx 1,500 ft/s
14th Century Hand Gonne, 430 gr .66" cal round ball at approx 1100 ft/s
From the 18th century onward, we see a trend of ever-increasing muzzle velocity and decreasing caliber. The introduction of paper cartridges containing Minié balls allowed the use of rifle-muskets as standard infantry small arms, as they combined the ease of loading of a musket with the accuracy and range of a rifle. The introduction of the Minié ball also saw a reduction in caliber: from .69" to .58", with a slight increase in projectile weight. This resulted in much greater sectional density, which contributed to the greatly improved ballistic performance of the new rifle-muskets. Additional tests after the American Civil War showed that .50" and later .45" calibers offered even more improved performance. Eventually, .45" caliber metallic cartridges in the form of the .45-70-405 Government would be the standard for the United States for nearly two decades.
What would replace the breech-loading Springfield rifle-musket would be a smokeless powder foreign bolt action rifle firing a bullet nearly half the weight at nearly half again the muzzle velocity, the 1892 Krag-Jørgensen. The Krag would prove to have a very short service life, being replaced by the superior (and also foreign) Mauser design in the form of the 1903 Springfield rifle, possessing extremely high performance and a muzzle velocity of 2,300 ft/s. With the implementation of aerodynamic spitzer projectiles in the 1906 rifle, velocity again improved to over 2,600 ft/s, and bullet weight was significantly reduced. The example cartridge used is the M1 Ball load, introduced in 1926, but higher velocity loads like M1906 Ball and M2 Ball were used extensively, both of which fired ~150 gr flat based bullets at 2,800 ft/s. Since the .30-06 cartridge comes in almost endless variations with different bullet weights and velocities, the M1 Ball type used here is intended to be representative of the caliber in general.
The self-loading era begins with the M1 Garand, also in .30-06, producing performance very similar to that of the Springfield or M14, with a 150 gr flat-based projectile fired at 2,800 feet per second. Because for the purposes of this discussion, it is highly similar to the M14, it has been omitted. After WWII, a program was initiated to develop a lighter magazine-fed select-fire rifle, and resulted, after over a decade of research and testing, in the M14, which was only an incremental improvement over the Garand upon which it is based. The M14, like the Krag before it, would see service as a standard rifle for less than a decade, though it would continue to be used in specialist roles up to the present day.
The next significant innovation in small arms was what is sometimes called "small-caliber, high-velocity" ammunition, but which in reality was just a continuation of a trend stretching back hundreds of years. US Army studies again suggested that a smaller caliber, lighter bullet, fired at a higher muzzle velocity was desirable, and after a period of development, the resulting 5.56mm cartridge was born, mated to the advanced ArmaLite rifle design. The ArmaLite rifle was unlike previous US Army rifles in many ways, perhaps not the least significant of which was the method of its development. Previous Army designs were developed by Army Ordnance, but the ArmaLite rifle was an independent development by a USMC officer-turned-Army Ordnance Colonel and an aircraft design engineer, in the employ of a division of Fairchild, an aircraft manufacturer. The original AR-10 design, in the same full-power caliber as the M14, competed in the light rifle trials, but was unsuccessful due to the exotic composite barrel, which burst during testing. The later AR-15 design, using the basic principles of the AR-10, but adapted for a much smaller .22" caliber cartridge at the request of Army Field Forces Board No. 3. The combination proved wildly successful, and a derivative design, the shorter M4 Carbine, remains the standard infantry weapon of the United States Army today.
Jumping backward in time, The "Kentucky" rifles collectively present an outlier, being much higher velocity and smaller caliber than their contemporaries. This was largely due to their tight rifling and sturdy construction, which incidentally made them unsuitable for use as a standard infantry arm, as they could not be loaded quickly. It did however foreshadow the improvements that would be made to rifle performance as rifling, Minié balls, and breechloading mechanisms allowed rifle-muskets to catch up to their civilian rifled cousins. The "Kentucky" rifles do not represent the state of military small arms at the time, but are included for completeness only.
A very strange anomaly can be seen at the bottom of the table, with both the 16th Century arquebus and 14th Century hand-gonne. Both have higher velocity than the later Charleville musket! Why is this? I do not know for sure, but I suspect that during this period, great emphasis was placed on defeating the personal body armor of the time. To do this would require considerable velocity using the bare lead round ball projectiles of the time. By the 18th century, all that was left of the heavy plate worn by infantry and cuirassiers of the 16th century was the gorget - a decorative neck-guard used as a status symbol. Without the need to penetrate heavy armor, and with no real ability of an individual to hit targets at beyond 20 yards with a smoothbore musket, velocity decreased.
Since the 18th century, the caliber and projectile weight of small arms have progressively decreased, while the muzzle velocity has progressively increased. The change from 7.62mm caliber rifles to 5.56mm caliber rifles follows this trend, and I expect sometime in the future that the 5.56mm caliber will be obsolesced by something even smaller, lighter, and higher velocity.
EDIT: Turns out, by pure serendipity, diving through the Weaponsman archives digs up a very similar article they wrote on the exact same subject a while back. Readers may find it interesting.