If we get the lower end of these projections, you might plausibly live to see a world with a billion people less than now, but to see a world with less than a billion total you’d have to have an extremely long lifetime, which implies some medical breakthrough, in which case, unless you have some super-exclusive access, the population would stop declining, just get super geriatric. I suppose there are other ways to do get there, such as global nuclear war, but even that might let quite a few people continue in the southern hemisphere.