Yeah, and all the best designers for dams and canals are from there, it's true. What your startlingly naive comment doesn't take into consideration is that it's ALWAYS been there, and the cities that we've built on the coasts in the last 50 years HAVEN'T been underwater. This is a new thing. They weren't designed for it.
But sure, take the coastal cities of the world out of the equation. The costs are still enormous, and still real. Agriculture, storms, unpredictable weather, weather patterns shifting substantially (snow where there wasn't snow previously, no snow where there used to be lots of snow), coral bleaching, ocean acidification, desertification...the list is really long. This is to say nothing of the stuff that we don't even know is coming; I suspect that we've failed to capture the entirety of the problem. The things that we ALREADY know about will cost a shit-tonne of money. The stuff that we DON'T know about are going to be even worse because it'll be impossible to prepare for them in any way.
Cost-benefit analyses really start to fall apart at this point.
The thing is, there are lots of little things that we can do, individually and societally, that don't cost much but slowly make a big difference. They've started adding sails to really big cargo ships. It's free energy. It helps. I walk to work, drive my car very little, and try to be good about my own personal energy usage. I use less energy now than I have ever before in my life. It wasn't a step down in my quality of life in the least. I live close enough to home that I can walk home for lunch now. I have fewer, nicer things.
Collapsing economies and cave-dwelling are a line that we've been sold by interests that have a stake in us not changing. I provide less revenue for oil companies than I used to. Because I pay a little more for better things, I don't dispose of things as often. As a consumer, I'm much less lucrative than I was 10 years ago.