You show again remarkable talent in picking out one little sentence while ignoring the rest of the post. The China comment was made as an illustration that your origin matters, it's clear to me that they can't split up China. [Though there might be some way if you do it via historical dynasty's instead of regions. In any case, that doesn't prevent Tibet or Manchuria as a DLC that's not available in China]
It's true, I probably should have brought up Arabia as the example of a civ that wouldn' exist if the game was made in Lebanon or Qatar. It's a better example than China arguably.
But other than that you fail to argue why population or trade matter and should be used as a basis for a quantitative comparison. I maintain that whatever factor you take to "rank" (and that's what you're doing) the empires, it will be highly subjective. For example, I personally don't have to think the Russian culture is interesting just because there are lots of them.
And btw. how can you argue for Aborigines and meanwhile say no to "Australia", last time I checked there are lots more "Australians" than "Aborigines" (of course the latter count to the former, but you get my point I hope).
Nigeria is the 7th most populous country in the world. There is arguably a national identity, just look at the success of the national football team. But would we want Nigeria as a civ? [To answer that, there might be some arguments for this, but most civ players in here prefer more "ancient" african civs.] And btw. by that metric, the Philippines would deserve inclusion before Vietnam or Burma.
Again, all I'm saying is that the "deserves to be included"-discussion is absolutely worthless. It doesn't further the discussion at all.
I don't actually say no to Australia, I just don't like the idea, much like I don't like the idea of Italy, Venice (who are now confirmed in), Canada, more Native American groups and such. As I've said even in this thread, if a colonial expansion were to be made, Australia and Canada would be decent options for a tie in to the expansion. I've also talked about an Indigenous Australian civ before, and in particular the issues that go along with one.
Population certainly isn't a particularly great metric, but in reference to others it is something useful to consider to gauge the size and significance of historical cultures. The Khmer for example were massive, not only on a regional scale either. Angkor at it's height was arguably on a similar level of any city before the Renaissance. The issue here, which I tend to not bother mention, is that they went for Siam "instead", which would be like going for France "instead" of Germany. The same issue exists for Mali, where they chose Songhai "instead" of them bizarrely as well. My guess would be they did it for a bit of shock factor in the build up to release rather than actually doing it for any cultural reason.
I also don't remember anyone talking only about modern populations. If you recall from the discussion about the Native North Americans and South East Asia before, the mention of modern populations was for flavour about the modern world, but the main reference was to them historically (although finding historical values for South East Asia directly is difficult). For a taste of the numbers though, the Khmer Empire in the 12th century had a population similar to the entire Native North American population today. On the other hand, whilst populous and united today, the Philippines did not have a Khmer Empire type power before the colonial period.
As for regional DLC, it's something I can't picture them ever bothering with. If they put all the time, expense and effort into making new Civs, they'd surely want them to be sold to everyone, and not limited by where they are "allowed".
Also, you're quite right about the effect that the origin of the game has. If the game was made (and dominated) by somewhere like Qatar or the UAE, it'd be unlike to have just an "Arabia" (although having an Arabia wouldn't be impossible culturally). With that example though, they'd have far more Civilizations from their own region, most likely with some that would seem odd to us. Personally I'd prefer to try and best to avoid being overly biased as the game tends to be to places related to America.