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.

....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.