I think the debate can be enhanced by reflection on our departing points on the type of uncertainty we are implicitly accepting in our models of the future. Do we assume that the future will be like the past? Do we assume that the future will not be like the past at all? How much novelty is in the system?
It is not only economists who have to deal with an uncertain future, anyone else, including environmentalists and politicians, to whom we may look for alternatives when discarding economic evaluation techniques, cannot escape an unknown future, and often implicit assumptions about future events.
Cost-benefit analysis plays an important role, even in the evaluation of projects and programmes with environmental implications. Cost-benefit analysis is no substitute for good decision making though. It helps, but is as bound by the uncertainty of future events as all other (often much less rigorous) techniques.