Or, space.
And we're paying to get that much mass (in the form of people and other requisite biomatter just to seed the habitats, if nothing else) out of our gravity well...how?
The current goal is the cost down of launching below a certain threshold, which I can't remember off the top of my head but I think it's like $2000 a kg? Either way, it's the point at which asteroid mining becomes profitable.
Then you can start to bring it down further by those mining companies pouring more into researching orbital refinement and construction, meaning people can work in space for longer periods and from more industries (since refined goods sell for more than unrefined).
The influx of resources and ability to construct large-scale constructions in space, coupled with the further advances in launch methods a large-scale industry would bring, means it's not an unfeasible future technology path.
I didn't say it'd happen soon, just that it's a path we can take that doesn't require artificially limiting human population. Refuting the suggestion that humanity would eventually need to limit it's population. There's enough resources in Mercury alone to last a loooong time, and we`re already starting to bring down the cost of space launches per kg.
I mean, eventually the sun will die and it'll all be pointless anyway unless we figure out interstellar travel. But, ya know. One problem at a time.