When creating this document, our goal wasn't to give people the impression that our populations should expand in number indefinitely (especially not on Earth alone). You're right that expecting the planet to shoulder indefinite population growth is impractical.

We're just trying to highlight the hazards of population decline that hits humanity like a tidal wave and produces unnecessary suffering and damage. There are ways for populations to sustainably decline without wiping out entire cultures, for example.

We, personally, care a lot about memetic/cultural diversity and worry that a population collapse for which we don't prepare will eliminate loads of perspectives that would otherwise contribute great new innovations and technologies to humanity's future.

We also think it's important that people know that population decline will affect certain fundamentals upon which their lives are based (e.g. "I put all my retirement savings in the stock market because on average the stock market always goes up" isn't going to be a healthy thought when population decline accelerates).