you are overestimating the damage caused by higher sea level and severely underestimating the damage caused by moving the freshwater saltwater line. I live in the Netherlands where we're basacally set up for sea level rises and severe storms that could happen once every 10000 years. so we have plenty of margin.

But the movenement of the freshwater/saltwater line is something we cannot control and it has severe impacts on fishing and the ecosystem now.