The whole world doesn’t need to. A sea level rise of 2m doesn’t matter for most of the world, or even for most coastal regions. And of the coastal regions that will be impacted most can be sacrificed with limited disruption. For the much more limited areas of the world that are coastal, have low elevation even well inland, and are densely populated or otherwise economically valuable, flood control is totally realistic (with the exception of some places like Florida that are screwed by the underlying geology).