It doesn't happen overnight. There is a gap between the moment a country drops below 2.1 and its populartion starts shrinking. US fertility rate fell below 2.1 between 2010 and 2015. It will take at least 3 decades until population peaks and starts shrinking.
Japan fertility rate dropped below 2.1 between 1975-1980, and as a result population grew and peaked until 2008, and from now on it will only shrink.
That is going to be reverted if we keep accepting immigration legally or illegally. I hope we all start understanding this now before it is too late. We should be giving incentives such as 10% tax cuts to those who have 2 or fewer kids over the age of 25 and start making women pay child support just like men and give them the same punishment if they don't pay it (jail time).