My point is that for cultures that never independently developed certain technologies, it is not "inaccurate" to depict them in Western dress. Cultures that adopted foreign technologies also often adopted foreign dress. Real-world modern infantry from Asian, African and South American countries that did not independently develop modern firearms wear Western uniforms, without any local traditional cultural elements. Man-at-arms and Pike & Shot are units that are peculiar to Europe; heavy infantry was mostly a Western thing, as was the tercio. I don't think it's unreasonable to default to Western dress for units that had no historical counterpart in the culture in question.
Creating speculative Aztec musketmen, etc., might be nice, but it would require a lot of creative license on the part of the artists (which is a lot more work than just depicting historical outfits), and it would be almost pure fantasy... and we know how much some folks here hate fantasy in their Civilization games.
(Also, people whining about free content.)
Completely Creative License is not the way to go and stay out of trouble. In fact, up until the Modern Era (late 20th century and later) when the very real problem of Being Seen = Being Dead put everybody into some kind of camouflage uniform and 'homogenized' everybody's troops, there were all kinds of variations in uniform, clothing, armor and equipment. The place to start is the colonial or 'native' troops raised by various European powers from the 17th century on: native/non-European gear and clothing with 'modern' European weapons.
What is needed is less Creative license and more Intensive Research. Such units would still be only theoretically 'historical' and would require a lot more work than simply translating historical still images to animation, but could be done and still be referenced to actual variations in most cases. In fact, the more difficult problem IMHO is 'backdating' Civs: what does an American Spearman or Swordssman look like given that America's entire history is post-gunpowder? We seem to have no problem moving right past that little problem, yet are excited about how to depict heavy plate armor on an Aztec . . .
The larger question is how much effort and resources do we want allocated to Unit Variations in the games? We've already settled for the Animated Leader Resource Sinks in Civ, do we demand something close to that in 3D animation (presumably not requiring the also-resource-intensive Voice Acting) for the Units? Or do we simply want a distinct visual differentiation by Civ/Faction amongst as much of the unit graphics as possible - while still being able to tell that, feathers or not, that little fellow is a spearman and not an Apostle of Huitzocatapultapect.
Again, research is the key: actual historical warriors and armies frequently spent a great deal of time finding ways to tell themselves apart, ranging from a distinctive sprig of plant in their hats or helmets to head to foot uniforms in distinctive colors and patterns to simply a distinctive flag, banner, symbol carried in front. Frankly, the fact that, as far as I can tell, neither Civ nor the new
Humankind game have done much with any of those possibilities in their unit graphics indicates that Unit Distinctions are not considered particularly important by the Design Teams.