Late Stage Capitalism

This sounds a lot like AI overlord islands proposed by MIT as one of twelve likely outcomes of out of control AI development

Extreme ideas like capping populations at a fixed number and further dividing people arbitrarily due to weird rules, people who have the same culture and language and identify as the same people but have different statuses and lifestyles due to their current governing body ... This all requires really strong authoritarianism, and a crackdown on human rights to control the growth of the population and a strong military to keep borders in check.

It would fucking suck. It just means more borders, less places you're allowed to be, stronger military police forces to enforce these things, and people getting in trouble for being pregnant or having too many kids or hopping a border to live with family they got separated from.

Yeah, that totally wouldn't backfire within about 5 seconds.

If you didn't notice, humans are super tribal. And that tribalism is strongly affected by any virtual border one pulls up to differentiate US from THEM.

Tribalism has degrees that determine just how friendly one is to another.

The largest tribe we have is us being humans. If there were aliens on our planet, most humans would sooner help another human or team up with one than an alien.

The next level of tribalism is generally understood to be religious. Religion gained this position only relatively recently on the human timeline, in the past ~2000 years (I'll let you guess why). Compared to earlier ages where every city state, every micro-region had an ever-evolving belief system influenced by its surroundings, today we have only a handful of dominant religions that spread vast amounts of people and areas, thus the re-categorisation on the tribalism scale. One will be much friendlier to someone of their religion than to someone with a differing religion.

Then we have the more geographical categorisations - national (which nation you belong to), linguistic (which language you speak), region (what region - this can be as small as a neighbourhood or as big as an entire country - you're from), and of course race and other region specific traits. And of course there's the cultural trait, which can again be as hyperlocal as specific to a town, or a county, or a country, or a continent...

This is already causing a lot of tensions in general. Just look at the Middle East - all of those countries, even though their religions are relatively close to each other, even though they're culturally similar, but the arbitrarily present borders and differing politics pits groups of people against each other.

The solution to this isn't to introduce even more arbitrary borders, especially not based on a random pulled-outta-my-arse population number. Look at e.g. London - a city of nearly ten million people. How do you split that to two groups of 5 million?

No, the solution is rather to create groupings of hyperlocal jurisdictions based on economic and cultural cohesion, with upward pressuring of larger groupings. Ensure all people are represented. Reduce the oppression of the majority (or in many cases, the plurality).

The main reason we have so much political tension (aside from the obviously ongoing populist propagandism spreading lies) is because in most political systems, a simple plurality can overpower any discussion or debate. You unite people on a single issue, tell them that all their issues are actually the same, and as long as that single issue remains, they'll provide a seemingly homogeneous block, while the rest try to get all their views represented, fracturing the representation.

The US is a perfect example of this on a large scale. The right managed to unite on what is essentially a single issue (but do see how groups are quickly ejected as soon as they try to even just discuss differences in opinion, you toe the line or you're a traitor), while the left would want some ideological fracturing, but end up shooting itself in the foot by doing so - ending up in a system where you're, at the end of the day, aren't voting for the person you truly want to represent you, but the candidate with the highest chance of winning, who can ensure that the worst candidate (in your eye) doesn't win. It's not progressive (as in, progressing your actual ideology, not the political direction itself) voting, but defensive.

What you're suggesting would just further this system of tribalism that hurts us as a global society.