The selection filter does not have to be total or near total for my point to stand, namely, Rule-of-Succession-like calculations can be useful even when one has enough information to think about the selection effects involved (provided that Rule-of-Succession-like calculations are ever useful).

And parenthetically selection effects on observations about whether nuclear exchanges happened in the past can be very strong. Consider for example a family who has lived in Washington, D.C., for the last 5 decades: Washington, D.C., is such an important target that it is unlikely the family would have survived the launch of most or all of the Soviet/Russian arsenal at the U.S. So, although I agree with you that the human race as a whole would probably have survived almost any plausible nuclear exchange, that does not do the family in D.C. much good. More precisely, it does not do much good for the family's ability to use historical data on whether or not nukes were launched at the U.S. in the past to refine their probability of launches in the future.