> We don't just shrug off the fantasy that there will be zero children born in "a couple of centuries"??

That's not a fantasy, it's the inevitable outcome of sub-replacement fertility, which is the state we find ourselves in (though my intuition says it will take longer than "a couple of centuries" to get to zero).

It's the inevitable outcome of everybody continuing it for all the generations that remain. As soon as there aren't enough people to manufacture contraceptives, it will of course grow. But after a few generations, there will be more land, water, animal and plant life, copper, cobalt, gold and such per person, and people can easily say "that shrinkage sucked, let's grow". You assume that things will always be the way they are now, which is of course false.

Yes. We’re saying that shrinkage sucked and it’s time to grow. It’s sooner than you’d imagined, perhaps.

....assuming the sub-replacement rate continues forever, which is a hefty assumption. It's quite certain that a greater-than-replacement rate can't continue forever (eventually, the mass of the humans would be greater than the mass of the planet), though that has been the world we've lived in up to now.

You should play a game of Age of Empires, and have your Villager population halved at some point of the game. See what happens then.

We do. Because as you said it's a fantasy.