In the Manhattan project, the "will bombs ignite the atmosphere?" question was analyzed and dismissed without much (to our knowledge) double-checking. The amount of risk checking per hour of human capital available can be expected to increase over time...

It's not much evidence, but the two earliest scientific investigations of existential risk I know of, LA-602 and the RHIC Review, seem to show movement in the opposite direction: "LA-602 was written by people curiously investigating whether a hydrogen bomb could ignite the atmosphere, and the RHIC Review is a work of public relations."

Perhaps the trend you describe is accurate, but I also wouldn't be surprised to find out (after further investigation) that scientists are now increasingly likely to avoid serious analysis of real risks posed by their research, since they're more worried than ever before about funding for their field (or, for some other reason). The AAAI Presidential Panel on Long-Term AI Futures was pretty disappointing, and like the RHIC Review seems like pure public relations, with a pre-determined conclusion and no serious risk analysis.