We are not talking about a rapid replacement of Europeans by third world tribal muslims -- not across the entirety of Europe. First, the replacement will occur within larger cities such as Marseilles, Paris, Amsterdam, Brussells, London. Since these cities are the seats of power for entire nations, the shock waves of population replacement will rattle the windows and shake the house frames across all of Europe.
It is impossible to overstate the psychological impact of an entire advanced but gelded civilisation being overtaken by one more primitive and aggressive.
Some European nations will survive -- as Israel currently survives surrounded and infiltrated by tribal people who hate the existence of the more advanced nation. Europe will become another battlefield of the "forever war." The war that has continued since the beginning of Islam, and that will persist until either Islam is gone, or there is no threat to the overweaning dominance of Islam.
When particular European nations -- and parts of nations -- fall to a more fertile, vital, and aggressive third world Islam, the rest of the (non-Islamic) world will look on in horror. Perhaps China will enjoy the spectacle, imagining itself immune from the type of suicidal immigration threat that Europe brought upon itself. It is unlikely that the Anglosphere that survives -- Canada, Australia, New Zealand, and the US -- will take the collapse of Europe lightly.
But then, the Anglosphere will have Islamic problems of its own -- brought on by dysfunctional immigration and suicidal political correctness.
Perhaps the real impetus for the "age of the seastead" will be a swarming locust-like dominance of Islam over the land masses of Earth, forcing the remnants of the civilised nations into the seas, to the poles, and into space -- if they are competent and prepared enough.