When arguing about the future, the imaginable is not all there is. You essentially gave several imaginable futures (some in which risks continue to arise, and others in which they do not) and did some handwaving about which class you considered likely to be larger. There are three ways to dispute this: to dispute your handwaving (eg, you consider compression of subjective time to be a conclusive argument, as if this is inevitable), to propose not-considered classes of future (eg, technology continues to increase, but some immutable law of the universe means that there are only a finite number of apocalyptic technologies), or to maintain that there are large classes of future which cannot possibly be imagined because they do not clearly fall into any categories such as we are likely to define in the present. If you use the latter dispute, arguing about probability is just arguing about which uninformative prior to use.