It's a series that garnered worldwide attention for its first season because it centers on a truth that's so obvious, yet so "inconvenient" for modern humanity to even contemplate.
This obvious truth is that when faced with the threat of "either you or x will die," people will act in the direction of survival at all costs. When the moment of life or death comes, those heroic clichés we're constantly told in the TV/film industry simply don't happen in real life. The survival instinct always prevails at the last moment.
This series' hallmark was portraying even its main character in accordance with the fundamental instinct I mentioned above, free of all sorts of clichés.
So why didn't people like the latest season?
The main reason for this dislike lies in the shift in theme.
The theme of this series' final season is diametrically opposed to the theme that fueled its success in the first season. This is the source of the sour taste in the mouth.
While the theme of the first season was that the limits of humanity's survival instinct are so "inappropriate" as to disturb standard social morality, this theme was suddenly discarded in the final season, and the same series suddenly became a clichéd Hollywood theme: "Humans are highly moral beings, humanity isn't dead, man." This transformation is, in essence, the source of the poor quality.
To achieve this finale, they added a baby, creating their deus ex machina. That was the cheapest part. Anyone with a brain knows you can't engage in something like infanticide, even on Netflix. So, we knew the baby would ultimately win, and we lost the element of surprise.
Not only did we lose the element of surprise, but for a baby to win this game, someone had to kill themselves. This part could have been handled, for example, the baby could have been left with the father for the finale. Here, logic was discarded at the expense of an arabesque farewell to our highly moral main character. Of course, there's character development, but in the first season, the man who defrauded a grandfather with Alzheimer's disease to survive and, in a way, sent him to his death, sacrificed his life in the finale to save the life of a child he'd never met.
To summarize:
The original and accurate analysis and critique of the system in the first season gave way to a morally superior protagonist. Things fell into cliché.
The tension of the first season gave way to a definitive flow. The tension, etc., has diminished to zero.
So what are we left with?
A mediocre third season, akin to a third-rate American action movie, with the main character ultimately sacrificing his life.