The effects are non-linear. What you see is that Bangladesh goes from being a habitable city one year to one which is underwater 1/4 of the time, look at the massive floods there this year. Next year might be dry, but the year after that? It is already not sustainably habitable, but the people still live there.

China (noting that most of their industrial cities are coastal) might have a bit more time, but it is going to mean rebuilding all of the infrastructure they have built--where do the resources come from for that?

And another question, before the people move, are they going to clean up all of their toxic waste? If not, it all goes into the water with the flooding, which then kills their uphill livelihood because they live next to a huge pool of poison.

Fuck, after the hurricane hit Texas, they decided to scrap environmentally sound cleanup because it was "too expensive given the scale of the loss." If the US can't clean up, why would developing countries?