Orbital habitats cannot carry passengers, [...]
I knew that they formally didn't carry such, but I was under the impression that they still provided their capacity automatically as if they were constantly inhabited.
Orbital habitats do not carry colonists by themselves so adding/removing habitats will not change the population of a planet. Should you remove habitats the people that were previously living there will simply be dumped on a planet. If the planet is like Venus and it does not have enough infrastructure the people will die. Quickly. If the planet is colony cost 0 nothing will happen.
One of the interesting things about orbital habitats is that they do not require any people working in agriculture and life support, which means that higher percentage of people can be working in manufacturing (player controlled installations). If you look at your population breakdown you'll see that the amount of people working in agriculture is 4.9% - which means some people are living in the habitats, as normally planet with colony cost 0 requires 5% of the population working in agriculture.
Well spotted, now I see that too. There is a miniscule shift of just a couple thousand people that equal 0.07% here - not what you would expect to happen with 10m habitat, but something at least. Now I feel like with a backwards Deja Vu, like I recognized that already sometime ago and just forgot about it.
So habitats really just improve upon the agri/social/labor ratio, and thus have lesser effect if that ratio is already naturally good in some place, like zero or even already <2 colonies.
To be honest it's kinda difficult for me to tell what did you expect from the habitats by installing them on a planet with colony cost 0, so I'm not sure if the above will help or not.
I expected 10m free fulltime mobile workers.^^ It would have been quite useful to just have a work platform with 10m guaranteed labor to be shipped wherever needed.
Strange that nobody bumped me on this yet, since I have posted 3 designs already with 5-20m habitats where I also described that planning.(..like running 400 fighter factories with 20m habitat alone..) Well, that was a misconception, but luckily I also ship around many more real colonists parallel, so nothing is lost.