I agree that we can (and should) shrink population.
But too quick a shrink is next to unmanageable. Suddenly empty towns, infrastructure that needs to be stopped before it's obsolete, simply because there are fewer customers, while you need to retrain many people to care for the elderly when there are not enough coming out of the school system to fill the positions, immigration that no longer takes the time for integration...
And people tend to feel happier when there are children around (particularly old people).

So a good fertility rate target is not >2.1 indeed, but it's not 1.2 either. I would hazard 1.8 as a good target. I guess Russia, Germany, Italy and Spain really are rather low and will suffer, particularly in the countryside.
France and the UK in particular are too high.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi