Let's suppose that the people that are involved with Geoengineering aren't the dangerous knuckleheads that I think they are and instead are actually capable of developing a system to fully control the atmosphere. Great! The status quo is preserved (which, as indicated by your earlier comment seems to be most desired). Life goes on, human population can continue to increase. But now we'll need more land for all these people and more food and more resources, etc. Sooner or later, and assuming something else doesn't change on us, we'll start reaching the physical limits of the continents. Well we can build out on to the seas until we run out of room there and so on. But guess what, we will inevitably find that our growth (our population size) is not unlimited. It seems only prudent to accept this sooner rather than later before we too become too big to fail.

As you've illustrated sea walls are not a viable long term or large scale option. But why should we even consider this? The need for sea walls is predicated on the fundamental assumption that the world is static, that everything is now what it will always will be. The whole issue of climate change seems to be more about realizing that we were wrong with this assumption than anything else (unless of course things don't have to change...).