So how much can existing coastlines be extended artificially before they start causing problems for other coastlines that are not artificially protected?
I won't calculate it but I would guessestimate that hundreds of km square of land reclamation from the sea would need to take place to make the sea level rize in any discernable manner. This is a non-issue.
More important is the question: where, when and how should we move coastal cities uphill, and when and how to protect them with dikes, or raise their level in situ.
Everything has to be evaluated in terms of scaleable sustainability over millennia. We can't assume that human development patterns that occur in the present will not continue growing for 1000s of years into the future.
We have to be able to forecast the future tectonic plate movements, what mountain ranges and continents will grow and which will shrink and how the shape of the ocean floor will change over time.
Then we have to make our developments and infrastructure fit within the natural processes of the planet, at all levels.
Humans have to get beyond the assumption that we can just build whatever we want and keep nature out. The Dutch were on the right path back when they used wind for shipping and milling, but I don't think damming the rising oceans are going to work as a solution for long-term sustainability.
Maybe there is a case to be made for it as a stop-gap measure until broader global sustainability is achieved. I'm just asking for transparency regarding any negative effects of such large engineering projects.