You are absolutely correct on the point that it becomes significantly harder to balance the more civs there are. In fact, it becomes exponentially harder. That’s because you need to consider the interaction with a new civ with every single civ that already exists.
This is really bad in 4v4 team games. If there were 8 civs and you added a new civ, this is 8 new interactions. If there are 43 civs and you add a new civ, there are 162409534560 possible new interactions that come with that civ. Lastly, if you introduce a new civ to 43 civs in a 3v3 setting, that is 115511760 new combinations. Obviously, you don’t deal with it this way because it is literally impossible. But it shows what the problem is.
Now, that being said, you should consider how arrogant and euro-centric you are being. You understand very little about history, but want to present some weird historical argument.
I think that there is a single factor we should consider if we to determine how important a civilization was. That is, how many people’s lives did it affect. If we consider all human life as being worth the same (which I think is a good moral principle), then this is the only thing that really matters. There are two angles to this, however. First, how many people’s lives did it affect during the age of the civ, and second, what legacy did it have and how many people did that affect. Since this is a historical game, we should give way more priority to the first than the second.
Now that the metric is set, it is easy to see that half of the European civs don’t matter, and there are tons of civs which are missing. Europe had less than 20% of the world population historically. Something ultra relevant to Europe might have been inconsequential to the rest of the world. China needs to split into multiple civs, India still needs more civs, and many of the european civs need to go.
But you don’t care about any of this. You only care about what is cool. So, be up front. Say “I don’t care about historical accuracy or whatever, I only want the cool civs around” and we can continue to ignore you.