It's not the sea-level rise itself that would be the most damaging economically long-term. It's the changing weather, and how it affects agriculture, water availability etc - and in some regions, the ability of humans to function there in general, for some parts of the year.
There is an incredible portion of the planet that is not habitable for most of the year. The majority of land on Earth is north of the equator, and very much of it is just flat out impossible to thrive in year round. Global warming will probably be a net benefit in this respect.
As far as agriculture is concerned, the IPCC says that rising temperatures will be good for farming (within a certain limit, 2-3c I believe). Carbon after all is an input to photosynthesis.
There's more to farming than temperature. A lot of people think that, as things get warmer, we'll simply move agriculture further to the north. But it's also a matter of soil fertility - many of the most fertile areas that we have today will be adversely affected, and the new ones that'll open up due to warming (e.g. in Canada and in Siberia) have very poor soils.
