If we must sharply curtail our greenhouse gas emissions or the Earth is doomed...then we are doomed. Either we have to sharply and deeply reduce the world's population (either actively by lining them up against a very long wall and shoot them or passively by reducing energy consumption to the point that massive starvation kicks in or some combination of the two. However it is to be done, we're talking about death by the billions) or we have to start considering alternative solutions.
Are you so concerned about global warming that you will be one of the first to volunteer for slow death by starvation?
Yeah, thought not.
So if we start looking at solutions that have any possibility of success whatsoever, then you are going to have to consider the fact that the only viable solution is "learn to live with it."
You said it yourself, the cost of mitigation only gets more expensive as time goes by. But technological innovation and the ensuing increases in efficiency mean that "living with it" gets cheaper and cheaper as time goes by.
The decision is not between good solutions and better solutions. The choice facing us today are solutions with no possibility at all of working and solutions that have a much higher theoretical (not to mention moral) standing.
Since I am not willing for anyone to die for my (possibly wrong) beliefs and certainly unwilling to kill for them, I find the decision an easy one.