This population growth will most likely continue AND
The problems caused by the population growth we already saw were acceptable enough. Who says we can't double it in the next 100 years? That'll be a slower growth rate than we've seen, and less costly to the average individual. Sure there might be problems of food, energy, water, etc. But that's not rising sea levels, and will happen regardless. However, the population will plateau at some stage, quite possibly during this century.
at the same time, the most habitable land will be disappearing.
Sure it'll be disappearing. But it's mostly only desirable because it's near the coast. If the coast moves then places that are now undesirable will become more desirable because they'll have easy access to the sea. Look at the endless sprawling underdeveloped wastes of space like Ohio, etc.
problem that will be caused by the destruction of many of these megacities AND the usual population growth,
What destruction? We're destroying our cities all the time. Look at the buildings around you. How old are most of them? 100 years? Unless you live in central London, that's very unlikely. Do you ever notice old buildings being demolished? How often to roads have to be resurfaced? Sewer pipes dug up and replaced? You're viewing large cities are static and irreplaceable. But they're mostly continuously being renewed.
