Replacement fertility rates assume zero net immigration, which does not hold for much of the OECD. For example, Canada's total fertility rate has stayed below 2.1 since the 1970s and has fallen since to its current low of 1.4 (https://datacommons.org/place/country/CAN?category=Demograph...), but its population has been consistently growing since then due to immigration: https://www.statcan.gc.ca/o1/en/plus/960-fewer-babies-born-c...