Why did the Romans use swords instead of spears? (A reply to Metatron)

Relief depicting Persian spearmen

Metatron, for those who don’t know of him, has made many excellent videos on YouTube regarding historical events and peoples.  He specialises in Japanese and Ancient Roman history, but regularly answers questions on other historical periods.

In one of his videos, he gives his view of why the Romans used swords instead of spears, as most warriors and soldiers did in antiquity.
He proposed that the Romans chose swords instead of spears because of the equipment of the opponents that they fought.  Most of them, he said were barbarians rather than the armoured men of later eras.  He rightly stated that the Romans would have quickly adapted their army to an armoured threat if they had to face one.  He gave an example of the Roman legions quickly employing the manica (arm guard) in response to the Dacian falx.

While he makes a good point about Roman adaptation to the threats they faced, the hypothesis that the Roman soldiers faced unarmoured troops throughout the majority of their Empire does not quite stand up to scrutiny.

From the late Republican era to the late Roman empire, Ancient Rome faced more extensive civil wars than possibly any other Empire in history.  Here are some examples:
The Social War

Sulla’s civil war

Caesar’s civil war

Series of civil wars involving Augustus, Anthony and the assassins of Julius Caesar

Year of the Four Emperors

The crisis of the Third Century (the largest, longest and most extensive civil war that effectively ended the Principate period)

The rule of Maximinus Thrax began the 3rd Century crisis

Civil Wars of the Tetrarchy

The statue of the tetrarchs includes Diocletian, Maximian, Galerius and Constantius

What each of these civil wars had in common was obviously that they pitted legionary against legionary.  Each side had heavily armed and armoured soldiers, with the large scutum, added into the equation. Moreover, the Romans fought a series of wars against the Parthians and various Persian dynasties such as the Pontians throughout the Republican and Imperial periods.  Dynasties like these had a great many troop types who would have been heavily armoured. 

While the Romans made some minor adaptions in equipment, such as strengthening the shield and helmet in response to the Dacian falx, their equipment and fighting style did not really change. 

They did not eschew the combination of javelin volleys followed up by sword and shield combat.  Given that the Romans were experts in warfare, we can conclude that it was an effective combination, no matter what kind of opponent they faced. 

It is impossible to assess their use of the short sword without factoring in the large Roman shield.  Roman troops did not fight in a shield to shield formation like the Greek Phalanx.  Each legionary had a fighting space of approximately one meter from the legionaries adjacent to him and behind him.  They would only use the testudo formation when facing a large volume of missile fire but would never use the formation for close combat.  The shorter gladius allowed the legionary to stab repeatedly from behind the shield into vulnerable places while having room to maneuver with their shorter weapons.  This was how the Greeks employed the xiphos if the phalanx formation collapsed.  Longer weapons in a tight press of bodies will be of little use.

The Greek Xiphos 

The argument that armour became progressively better was the reason for the abandonment of the sword is not borne out by looking to developments of the middle ages.

For the early middle ages, the sword was the prized weapon of the elite fighters such as nobles or the household guards.  But these men would also have had access to high-quality mail armour.  The best troops were often employed to engage the best troops of the other side.  Given this fact that the sword continued to be the prized weapon, we can assume that it gave even greater tactical flexibility compared to other weapons such as spears and axes. 

In the later middle ages, plate armour became increasingly common for all types of soldiers.  Polearms such as the halberd and pike became more common than in previous times due to the greater presence of heavy cavalry.  The sword, however, was certainly not abandoned.  They were still common for elite warriors and soldiers on the battlefield.   The profile of the weapon changed.  It had become thinner than the earlier migration period sword that predated it.  This enabled it to be better thrust into the tighter gaps in plate armour, rather than through the armour itself. 

Migration period swords gradually grew long and thin

In conclusion:

The sword and shield combination was eventually abandoned, but not for reasons relating to an increase in the use of armour.  Gunpowder weapons ensured the demise of many weapons and tactics of previous times.