Another option is an involuntary contraction caused by the effects of climate change, as fresh water supplies dwindle, soil erodes, and ecological collapse causes famine, war, and death. We are, say, 5% along this path.
In the worst case, technology can surely sustain at least a small human population indefinitely, even on a decimated planet Earth - dome living, and so forth. You know, the kinds of things we once imagined for settling distant planets. With any luck they will be scientists and engineers, working diligently to find a path back to our historical equilibrium that supported so much diverse life for so long. But that seems desperate, and hopeless, since achieving breakthroughs seems to require a certain population density and technology level.
What is striking to me is how reckless Americans are in consumption. Having lived in Germany a few years, I'm struck by how much more efficient and frugal life is there. Public transportation is easy and ubiquitous, healthcare is rational, simple, cheap, yet of high quality. "Stuff" is certainly more expensive, and you have fewer readily available options. People repair rather than replace. Many don't own a car. America could at least aim for something like that as part of a "voluntary contraction"! But I think we will continue to live recklessly, stuffing our bodies and homes and minds with junk until we die, bloated in every possible way, wondering why the world hates us for being free.