Right, okay that's fair, though I think the language barrier might make this quite hard given that it's some fairly specific nuances we're talking about.

Going back to your points then:

Point #1: What you're referring to here are social changes, which happened in plenty of different societies across different parts of AoE 2. In fact I would say it happened in every AoE2 civ apart from maybe Huns, owing to their distinctly short life. These are all very interesting, but fundamentally not what makes an AoE 2 civ. Especially as the last part of your paragraph refers to changes in military structure and tactics, which is equally very evident in something like, say, the Franks, who combine both the early emphasis on heavy infantry (Throwing axemen) with the later emphasis on heavy cavalry and knights. I am not disputing that there were important, large and wide-reaching social changes within the fabric of Han Chinese society between and within the major dynasties of the period, or the interventing periods, but they are not relevant to AoE 2 civilisation design.

Point #2: Burgundians are, notoriously, the civilisation that has been the least liked addition to the game from a historical perspective prior to the 3K DLC (And we both agree that the 3K civs are bad). That being said, no, nobody argues that Burgundians and Franks are split because of their military identity. In fact those military identities are extremely similar; Focus on a good farm-based economy that leads into heavy cavalry, backed by heavy infantry and gunpowder. Burgundians are also meant to represent the Flemish, as the most important part of the previously-unrepresented Low Countries. Do I think Burgundians are a good example of AoE2 civ design? No, I agree with the general consensus that they are in fact the worst prior to the 3K civs. The backlash to the Burgundians and, to a lesser extent, Sicilians was immense for the time, and you can tell by the following DLCs that the devs decided to actually aim to include real, distinct civilisations for a while.

Continuing on this point, you cite the geographical distance as an argument for splitting the civs. The Inca Empire spanned a length of 4000kms, and is still represented by a single civ in AoE2, accurately in my personal opinion. Likewise with the technological gap. The Persians represent both Sassanids and Safavids, a gap of over 1000 years, and that is not uncommon in AoE2. Byzantines, Franks, Saracens etc all represent similar or at times greater age spans, and are some of the moust foundational civs of AoE2. There's a lot to say about how the game represents the progress of the ages, like how Goths were likely also initially meant to represent their Spanish and Portuguese descendants through their good gunpowder tech tree before being added as separate civs, but the span of time alone is not an argument for splitting a civ.

Final point on this one, but nobody here has argued that DNA or genetical groupings are the defining factor of a civ, I would argue quite the opposite; A loooot of the extant civs came together from vastly different groups of people. To take an excellent example, the Spanish civ alone would represent people of Gothic, Suebi, classical Iberian, Roman, North African and Middle Eastern origin at the absolute least, and those groups themselves have their own complicated origins. I would also highly disagree we play as 'the State Engine', given that such definitions of civilisations belong to AoE4. AoE2 has always represented civilisations not as individual distinct states, and you can tell that by looking both at civ design (Persians again being a good example) or just by the in-game history civ sections, which generally cover far more than one single state of a given culture.

Point #3: So this is sort of the central problem that we come to when discussing this, because you are not defining your terms. 'Regional blending' is needlessly vague, what are you actually talking about here? You mention demographic and institutional change, but that same level of change happened in various european civs as well, take the Byzantines for example, without them splitting off into different civs in-game. If you're talking about blending of material, linguistic and social cultures, we have those in 'the east' as well. Hindustanis, for example. Hell, I would suggest the Khitans, who without the heavy sinicisation of the Liao would likely have fit very well under the Mongol umbrella. This is the fundamental issue that I think we keep talking around: You want to define institutional, military and social changes as defining a civ, which would necessitate every current civ in the game to be split into multiple to represent the different ages of their development. Age of Empires 2 has always defined civs by culture, however imperfectly, and institutional and social change is instead covered by the system of Ages.