We went from a hundred million to eight billion in less than two hundred years. It is only relevant for the next 80 years.
Birth rate only determines the time it takes to go from one population number to another, it doesn't change the fundamental problem of carrying capacity. The carrying capacity of Earth is a trillion times higher than Mars. That isn't even enough to let a single human live on Mars.
If you exceed the carrying capacity on Earth all you get to do is send people to die on Mars instead.
The big filter in terms of population is our ability to terraform other planets and since terraforming takes thousands of years, staying alive on earth with stagnant birth rates is an important part of it until the terraforming is complete.
> The big filter in terms of population is our ability to terraform other planets and since terraforming takes thousands of years, staying alive on earth with stagnant birth rates is an important part of it until the terraforming is complete.
I think this is actually very crucial, and something too few people appreciate: We need (or will need, at some point; the carrying capacity of the Earth is actually quite a bit more than 8-10 billion, we just need to be better about distributing resources) to keep population from continuing to grow forever on Earth until we have reliable means of colonizing other planets.
This also means we need to avoid getting too far in life-extension/physical immortality research before then! If we can actually achieve physical immortality, but we're stuck on Earth, at some point we will have three choices: stop having kids, start killing each other off in significant numbers, or start running out of resources.
(That is, of course, entirely separate from the concern that life-extension treatments will only be available to the very rich.)
The big filter in terms of population is our ability to terraform other planets and since terraforming takes thousands of years, staying alive on earth with stagnant birth rates is an important part of it until the terraforming is complete.