The reason is that widening replacement-level fertility means population growth is slowing down anyway.

ECONOMIST: Lower fertility is changing the world for the better

In China and most of the industrialised world, births and deaths are at or below replacement level (though America's population still rises because of immigration).

ECONOMIST: Unshapely world, too old or too young

Still, because the current level is far below the level of 2.07 required to prevent population replacement, the result is net decline.

FORBES: Japan's Demographic and Cultural Destiny