Well, medieval swordsmen didn't differ from classical swordsmen significantly. And although there were a lot of iterations of handheld firearms units, the borders could be drawn in different way. Separating on the point of moving from muzzle-loading weapons and fancy uniforms to breech-loading weapons and khaki is the most significant one. If they needed to decrease the number of upgrades, they did it right.
Actually, no they didn't.
Right now they (apparently) have the musketman-type unit and the Infantry. The musket man seems to equate to the matchlock musket or arquebus.
Let's do a comparison.
The matchlock musket had an effective range of about 100 yards, could be loaded and fired (continuous fire) about once a minute, and, because the musketeer was waving around a lighted match, the men had to be several feet apart to avoid the danger of setting each other on fire or worse, the powder charges they were carrying.
The Fusil, or firelock - the flintlock smoothbore musket that replaced the matchlock, had the same range, but could be fired 2 - 3 times a minute and, not having lighted fire on their persons, the fusiliers could be shoulder to shoulder. Also, virtually simultaneous with the introduction of the flintlock, the socket bayonet was introduced, which gave the musket-wielding infantry a good melee weapon (bayonet-armed men could and did take on swordsmen and spearmen and defeat them) and protection against cavalry.
So, between the two the flintlock or Fusil gives at least twice the shots per minute and twice the shots per yard/meter of frontage, better melee, anti-cavalry bonus but the same range.
The black powder breechloading rifle allowed up to 5 - 6 shots per minute and an effective range of up to 3 - 400 yards - if the black powder smoke allowed you to see any targets: research on real firing ranges during the American Civil War revealed that most firefights still took place at less than 100 yards' range - the same as with the smoothbore muskets of the Napoleonic Wars!
So, the 'rifle' gives at least a doubling of firepower over the flintlock smoothbore and approximately a doubling of the range.
The real change comes less than 40 years after the breechloading blackpowder rifle, with the introduction of smokeless powder and the magazine-fed rifle (Dreyse Needlegun, the first officially-adopted breechloader was 1859, the magazine-fed Mausers and their like were introduced in the 1880s)
The smokeless powder weapons (called 'Great War Infantry' in Civ V, an appallingly bad choice of title for the unit) could fire up to 20 aimed shots per minute to ranges of 400 - 800 yards. Again, the range approximately doubles over the black powder rifle, but without the obscuring smoke and the bullets flying per minute triples.
Now, last point:
The matchlock musket was in general use for about 150 years, until 1700
The flintlock or Firelock musket was in general use for about 150 years, 1700 - 1850
The breechloading black powder rifle was in use for about 25 years: 1860 - 1885
The magazine-rifle-armed infantry persisted for about 50 years: 1885 - 1935
After 1935 you start getting 'modern infantry' (well, World War Two infantry), in which light machine-guns or automatic rifles supplement the rifles down to squad level, and the battalions and regiments have mortars, heavy machine-guns and light cannon or antitank cannon in them.
So, the biggest changes come
Between the 'Musketman' and the 'Fusilier', when the effective firepower doubles AND the melee value increases AND the unit gets some anti-cavalry bonus (bayonet)
And
Between the black-powder rifle and the smokeless-powder rifle, when the firepower approximately triples and the range at least doubles
BUT
To leave out all of these except the matchlock musket man and the 1935+ infantryman is total nonsense.
If for no other reason, both the British/English and French 'Unique Units' - the Imperial Guard and the 'Red Coat' - were armed with the flintlock musket, which has NEVER been depicted in Civ games!
After 6 tries, you'd think they'd get it right, but apparently, it will take at least 7 attempts, or more...