Ultimately it is the only option. What flood control measures? You don't even have a clue what will be required, you just have blind faith it will happen.
It is certainly more rational than your blind faith that CO2 mitigation will accomplish something useful.
Remember that I am not claiming to know that adaption will work. I am only claiming that the chances that adaption will accomplish something useful are many times greater than the chances that CO2 mitigation will accomplish something useful.
Lets take flood control: we know how to build dams and dikes. We can pay to move people from high risk areas. More importantly, every dollar spent on dams or dikes or relocation will incrementally help some people.
OTOH, we can spend the same amount of money subsidizing wind and/solar or electric vehicles and reduce CO2 emission by some fraction of a percentage. All that money helps no one because the effect will be too small to matter.
To make matters worse, when we finally realize that adaption is the only option we will have made the cost of adaption higher by making it more expensive to run the heavy equipment and produce the concrete needed for dams and dikes. IOW, CO2 mitigation policies will harm more people in long run because they hinder our ability to adapt.
I understand the instinct to try and fix the problem at the source but in this case your instincts are simply wrong. The better strategy is to deal with the consequences as best as we can.