There is a 4th scenario: the population peaks and begins to fall. The article talks about how growth rates are falling all over the world. Many countries already have birthrates below replacement level, and the world as whole will reach that by 2085, according to UN projections. People living longer is a countervailing force, but declining population is still a pretty likely future.

(Can't link to the specific stat, but take a look here for raw data and projections https://population.un.org/wpp/DataQuery/)