> > Somehow in the 1970s we got by with 1/2 as many people as we have now - why would a long, slow decline back down to that number be catastrophic?

Because we've built an economic system based on the idea that you must, at all costs, grow. Stable success is insufficient and will be seen as failure.

And then we spent many decades programming people to believe this is the only thing that can possible work.

Nothing natural about this at all, it is just the result of the cult of capitalism.

Have we, though? Governments rely on economic growth to fund their plans. That means economic activity has to grow, which means ... something ultimately rather unclear about what people around at the time consider wealth to be. It doesn't mean "everybody into the spawning pools and get busy".