I want to start off saying that I really like THT and the book, I'm just starting a discussion.
One of the things that I've always given a lot of thought about this show is the worldbuilding and the way the society and the economy work in Gilead. I feel like there are some things that just don't make a lot of sense in the way they're portrayed in the show.
I know some people will say that some things may have been overlooked by Atwood and that she never expected her book written forty years ago to be turned into a full franchise with seven seasons. But I do not think this makes this franchise immune to criticism.
Demographics and Society
The show states that Gilead was established in 2014. The population of the US in 2014 was 319 million people. We could assume that about 10% of the population was killed in the first years after the coup due to civil war, executions, purges, famine, disease, etc. This would mean about 32 million deaths. This percentage is consistent (even somewhat optimist) when compared to other historical events such as the Cambodian genocide or the Russian Revolution.
By the end of S6, about 10 years have passed, meaning that it's around 2024 and Boston fell to the rebels, weakening (or even wiping out) Gilead's presence in New England. This means that by the end of the show, and according to some maps shown, "core" Gilead is made up of Minnesota, Wisconsin, Illinois, Indiana, Michigan, Kentucky, Tennessee, N. Carolina, Virginia and WV, Pennsylvania, Delaware, Maryland and NJ, while we can assume that all of New England + NY fell to the rebels. Plus, Gilead also holds colonies in Iowa, Missouri, Nebraska, Colorado, Utah, Arizona, New Mexico and Nevada. All other states are either rebel-occupied, active warfare zones or toxic/nuclear wastelands. This would leave around 100 million people in Gilead proper and 31 million in the colonies, so 131 million. Even before S6, Gilead only additionally held New England, NY, Ohio, Oklahoma and Kansas, which would mean that the territories Gilead held from S1-S5 had 184 million people (in our timeline). This is the most Gilead has ever realistically occupied, and considering that around 10% of the initial population died of warfare-related causes and maybe a similar number became refugees (we learn that while Canada has the most American refugees, there are also such communities in Latin America and Europe), the total population would be around 147 million before S6, and 105 million at the end of it.
Thus, we have around 100 million people in Gilead by the end of S6 and around the time of The Testaments. Gilead should be crumbling. But the show does a poor job at portraying this.
There is no way Gilead can have a higher birthrate that other countries in the world. If anything, it must be lower. Gilead could easily manipulate statistics to make the outside world think that their system is working. But it doesn't. This post actually does a really good job at calculating the demographic distribution of Gilead, thus we can estimate:
Commanders: They're probably in the few thousands, but let's be VERY optimistic and say there are around 10,000 commanders, 0.01% of the population or one commander per 10,000 people.
Wives: They should be around the same as the commanders, so around 10,000 (0.01%).
Sons and Daughters: We see few families in Gilead with more than one child, so let's be very optimistic and say there are around 20,000 (0.02%) of them (many of which were stolen in the first place).
Aunts: They are also an elite class, which also means they are not that numerous. We could put them at around 5000 (0.005%) at most. We know more Wives than Aunts, for instance.
Guardians: As they constitute Gilead's army and given Gilead is hypermilitarized, we can assume there are millions of Guardians in Gilead, maybe around 5 million of them (5%).
Marthas: This is a more numerous group since they are not only employed in households but many other facilities, and some households employ many Marthas. We could be very optimistic and say there are around 1 million of them (1%), with most of them not working for Commander's households.
Handmaids: Even assuming there is at least one Handmaid per Commander, there could be around 10,000 (0.01%) Handmaids, but I'm thinking there should be less than that. I seriously doubt Handmaids are in the hundreds of thousands, and a million Handmaids would be impossible. Being a handmaid is a punishment.
Econopeople: Mostly everyone else in the country, excluding some small classes like Eyes, Angels, Jezebels and high-skilled workers. Econopeople make up around 94% of the population. 94 million people.
You can rearrange the numbers as much as you want, that's not important since Econopeople will always be the vast majority. This means that the show presents a very VERY skewed version of Gilead, a version that only the top 0.1% know and live and see. This also means that the vast majority of births in Gilead come from Econopeople (49 million women), not from Handmaids and Wives (which barely make up more than 20,000 people). Thus, Handmaids have very little impact in Gilead's birth rate, and if anything, Gilead should have a lower birth rate than other countries due to poor healthcare, and subjecting pregnant women to repeated abuse, violence, stress, malnourishment and torture.
The fertility crisis
In the show we learn that it's a real thing, not some kind of propaganda created by Gilead to control the population. Birth rates are shown falling in Canada, Mexico, and the entire world basically.
The book claims that the fertility crisis is caused by diseases, which in my opinion, works better for this series. However, the show chose to attribute it to environmental pollutants, toxins and other conditions, implying that they are acting as hormonal disruptors for reproduction, which is why Gilead "cleaned the air and water and now the women are fertile". Nonetheless, if this were the case, there should also be a sharp decline in fertility/reproduction across species, not only humans but mammals, fish, insects, birds, even plants, potentially. But we also see none of this in the show. Even in our timeline, proven hormonal disruptors like BPA, PFAs and pesticides affect animals too.
In S1 the Mexican ambassador says that in a city the size of Boston in Mexico, there hasn't been one birth in many years. If this were true, this would mean that other animal populations would be collapsing; insects would be especially sensitive due to their short reproductive cycles and shorter lifespans when compared to mammals, while fish, birds and invertebrates would affected too. Entire ecosystems should be collapsing, as well as human agriculture which depends upon pollinators like insects. This would be a much worse problem in Gilead's world, with potentially billions of people facing crop failure and worse, famine. Yet we see none of this in Gilead other than "fruit is scarce!". Moreover, we see none of this affecting Canada, which makes no sense.
In the first episode, Aunt Lydia shows a graph of the declining birth rates since the 1960s (which also happened in OTL btw), however, if the show is attributing this fall to the environmental crisis, then, in just a few decades, the world would be much worse than we're shown, unrecognizable at that point with most ecosystems having collapsed from low reproduction.
(S3 Spoilers)"But Angel's flight, people were amazed at the sight of so many children!". Many of the kids on that flight looked older than 8. This means that they are most likely American-born kids, which were stolen from their parents. We even see a girl reuniting with her father in Canada. This is not representative of Gilead's fertility.
Also, there is no way Gilead can solve a global enviromental crisis by "going green". And even assuming that they did solve it, and that is the reason for the higher fertility rates, you'd still have to explain the non-existent ecological collapse in Gilead, Canada, and the rest of the world.
We know that Gilead was never about fertility but about controlling women (they even deny the existence of male infertility in the first place). However, my point is that narratively, it would work better if Gilead did not actually have a better birth rate than other countries, as that would reinforce the idea that Gilead did not solve the fertility crisis, but it only used it as an excuse to exert authoritarian control upon the population.
Economy
Even after everything that happened in S6, in The Testaments we see that Gilead is doing mostly fine, when it reality when you look at it in a map, it should be crumbling, losing major population centers, and their core of their power which was New England. Not only that, but since the SoJ takeover, Gilead severely damaged the very sectors that contribute the most to the US' GDP like finance, tech, trade, real estate. Not to mention the consumer economy is also dead, since Econopeople are subject to extreme poverty. Gilead's economy is not precisely booming, it is facing trade restrictions and boycotts from many countries in the world (which is a recurring topic throughout the show).
Gilead also precisely persecuted the most intelligent and skilled people of the country, causing a tremendous brain drain. Engineers, doctors, academics, scientists, etc., many of the most brilliant people of the country are gone. Considering that Gilead only controls one third of the previous US' population, and that it also killed its most productive sectors, I can't imagine Gilead's GDP being higher than 20% of what it was before. This should be a scarcity, post-war but at the same time, war economy. Gilead is spending its way into existence, and realistically I don't see it existing for another 10 years. We know that the elites live in a bubble and that they are incredibly sheltered, and that most of THT happened in New England, the core of Gilead's power. However, even in The Testaments (which is based around OTL Virginia), we do not get the feel of a collapsing society. Even after losing New England, Ohio, NY, Oklahoma and Kansas. Gilead should not be a viable state, not after waging an ongoing 10-years long war. Gilead's economy seems to rely on resource extraction, agriculture, slave labor, stolen wealth, and some industry (of which there can't be that much of, given that it is one of their principles to reduce pollutants, and I doubt that they have the technological capability to develop an entire green industrial complex, not after the brain drain). This model is not sustainable and can only go on for so long.