> I wonder how Island, Greenland, Norway etc. live with a population count far lower without the fear of extinction.

Countries aren't species, they are social constructs; they are maintained memetically, not genetically. There is no shortage of humans to assure that survival of countries that pay sufficient effort to their memetic continuity.

With present population trends, in a few centuries it might be time to be concerned about birthrates if they don't adjust thermostatically to changed global population conditions, but there is certainly no imminent threat from low birthrates.