Some people think Earth can't support a billion people.
I think that is simply wrong.
But 10 billion people, that getting up there. And at 50
billion that would cause me some pause.
But suppose we simply wanted planet Earth to have
1 billion or less people- because that's what we wanted.
It would nicer in some way.
I think it would be possible to get to planet Earth which
had 1 billion or less- and it doesn't involve killing large numbers
of people or some kind weird State population control.

In the past when I've considered the future, I think of humans going
into space and using Space resources and having a much larger
earth population than the present. And really no limit to human
population living on Earth, so with energy and products being made
in space, I usually imagine an earth population of 25-50 billion and
of that somewhere around 5 billion living in Earth orbit and no real
limit in terms of human population beyond Earth- say 1000 years in
the future and having a total human population of somewhere around
100 trillion- and of course most would not be living on Earth.

BUT suppose that the majority of people did want a low population on
Earth, it seems to me if this was actually desired, then opening the
space frontier would allow this. Whereas, I don't think it's really possible
do this with any kind of mass slaughter of humans and/or a population
control- because human are capable of rebuilding population levels over
time- and considering how well govt controls anything, it's rather foolish
to imagine them controling human population for any extended period of
time.

Now, once humans are in the space environment, opposed to being in a
situation of needing to lift everything needed up the gravity well of Earth,
making things in space should cheaper than making things of Earth. The reasons
why it would cheaper are numerous. The main reason is that Space is like an
ocean- an ocean with relatively minor weather problems and far superior in other aspects. And if you look at Earth most of human populations and commerce is near an ocean or river. So mainly cheap transportation. Another aspect is ability to produce cheap energy. The Sun is a giant fusion reactor and solar power at Earth distance is at least 2 times more efficient. Going closer to the sun, would increase this efficiency further, say 10 times or more. But you would just be limited to solar power. Nuclear power [fission] is simply a completely different game in space- being 5 or 10 times cheaper than compared to using it on Earth. Also Nuclear fusion could very well be something which could be more easily developed in the space environment [there is no shortage of vacuum]. And real estate would be fairly cheap. Now, some locations could high real estate value- Earth orbits, some locations on the Moon, etc. But if you just want say cubic mile of space millions of miles from earth, that would be cheap, because there are a lot of cubic miles of space millions of miles from Earth. But suppose you want a more prime location of say 1 1/2 millions from Earth at Earth/Sun L-1 point, the volume of this space is several cubic volumes of Earth. Anyhow no shortage of real estate.
And as far as stuff: gold, water, or whatever, there is far more stuff in Space than on Earth. In the main asteroid belt there is more water than on Earth. More metals, more oxygen, carbon, etc. The only thing in our solar system which there could a shortage is life. Oh, and vacuum is useful for manufacture of many things- obvious things like computer chips and solar panels, but a lot of other things also.

So, this is not new, the problem is how to get off earth, cheaply.
This isn't really a problem. I don't think we have to invent any new technology to do this and that it is quite possible to do with the chemical rocket technology we have had for over 40 years. I would say that basically we are already on the path of doing this- this path may take us a few decades or longer to get to somewhere near to this point.
If we wanted to quicken this process and used some intelligence this path could be shorten considerable for modest amounts of money.

So, getting back to my point that opening the space frontier could give the option
of reducing Earth's population [assuming this is desired]. Now, I think what could easily be seen as desirable is to move a large percentage of industry on Earth to the Space environment. Now, govt incentives could used to accelerate this process, but I also think there would eventually be a natural tendency for this to happen [because of the economic advantages given above]. Govt incentive could be in form of tax breaks [e.g. Zero space tax].
Now, for instance if you had a mature industry in Space there is no reason why consumer products for earthlings couldn't delivered to their doorstep anywhere in the world in about 30 mins. And it conceivable this could include such things as pizza delivery. Space after all is just 50 miles up, and in terms of energy cost to delivering it, it could easily cost less than delivery person driving 20 min somewhere. Though something in near space over an urban area might do it cheaper than directly from orbit- but whatever is in near space could supplied from orbit. But for those on a boat in the middle of Pacific desiring pizza- from orbit may be only option.
Now, if you have a large amount industry in space, and people want jobs, there might be a natural tendency for people to be somewhat close to their job. Therefore in this environment, it wouldn't seem very difficult for govt to encourage people to move off world. High taxes should be quite effective, if homes in space are cheap and taxes there are fairly low.