There are steps between doing nothing and abandoning the east coast. There are some areas (right at sea level, river flood zones) which probably are better off as parks, and letting things like costal mangrove forests regrow would be important, but there’s a lot of places where a significant fraction of the existing buildings could remain viable if they were sensibly built following good design practice rather than as cheaply as the developers could get away with.

It seems like something flood insurance and building codes should get increasingly stringent about, similar to how some California fire codes started requiring houses to be built to support sheltering in place during wildfires so you didn’t have so many people evacuating and needing to rebuild as many homes. It’d be nice if that started as guaranteed annual premium increases for unsafe properties with an assistance program for primary residences which phases out based on property value.