We could easily adapt to the maximum amount the seas will rise if we do nothing.
At worst it will be about 200m. It sounds bad, but most land is above that, and there is enough flat land to use then.
All we need to do is start moving populations from the areas most affected, (most of Africa, the Middle East, India, Mexico, the American midwest and coastal areas, the whole of Southern Europe, Indonesia, and any European city near a coast) to locations that will become usable. Northern Canada and Northern Russia are vast areas of flat land that will become usable then.
Not to mention Greenland and Antartica.
Sure, there are no cities or infrastructure there, so we would need to build that. And the land is currently unable to support agriculture, so we would need to ensure the soil becomes useful. And of course we'd need to integrate all these different cultures. And maybe the governments currently owning these areas might have a different opinion.
But if we all pull together we can do that, after all it's not like mass migrations due to environmental changes in the past ever led to problems.
So let's not hurt today's economy by being careful, as that would deny humanity of the future the joy of such a challenge.
Well said (for those whose sarcasm detectors are working, at least)!