"how improbable does a cataclysm have to be to warrant proceeding with an experiment?"

The same concerns, thankfully unfounded, where expresssed prior to the first tests of fission and fusion weapons. It was feared runaway chain reactions could could destroy the earth. Realistically, as our science and engineering begins dealing with higher and higher energies at fundamental scales, micro and macro, this sort of scenario becomes more plausible.

Perhaps out in the universe somewhere, a shock front of complete space-time oblivion generated by some industrial accident somewhere is rushing towards us at the speed of light.

Perhaps the reason the universe is so large, and the speed of light so slow, is that this is the only way the universe can survive long enough to be interesting before its own habitants destroy it--or to ensure enough time passes so a species can evolve capable of doing so.

Perhaps the only reason intelligence evolves at all is so that the universe can periodically annihilate itself and start over.