You can't just assume away human capability for adaption.
If we have 1000 years to prepare for it, it most likely means that we will gain several miles of coastal land as we discover the best places (which may well be some way out to sea) to build dikes around the (relatively few) places where it's impractical to just move the population.
The Netherlands are often mentioned as the first place to go, but that's disingenuous: Nowhere else is better prepared to slap 50cm each century on top of their existing dikes. No, it's not quite that simple, but it's orders of magnitude simpler than constructing building a dike from scratch around, say, New York City.
At this rate, 50cm is not going to be enough. And you don't slap it just on top, you have to widen the entire dike to make it stronger.
No, we won't be the first place to go; that'll be the Maldives. And we probably can afford the extreme expenditure necessary to increase those dikes. But wouldn't it have been better to prevent that extreme expenditure in the first place?
No, sorry, 50cm per 100 years is too little. It should be 70cm.
The "extreme expenditure" to build dikes is a lot less than the expenditure to not just stop, but revert global warming. Nobody knows if it's even possible, so we might end up wasting all that money and then still having to build dikes.
If we have 1000 years to prepare for it, it most likely means that we will gain several miles of coastal land as we discover the best places (which may well be some way out to sea) to build dikes around the (relatively few) places where it's impractical to just move the population.
The Netherlands are often mentioned as the first place to go, but that's disingenuous: Nowhere else is better prepared to slap 50cm each century on top of their existing dikes. No, it's not quite that simple, but it's orders of magnitude simpler than constructing building a dike from scratch around, say, New York City.