The huge problem with any probabalistic estimates is the assumption that repeated failures of the LHC are independent (like coin tosses) and infrequent. Why, in an immensely complex bit of novel engineering, with a vast number of components interacting, would you assume that? How many varieties of filament failed when Edison was trying to build a light bulb? Thousands: but that did not prove that the carbon filament lightbulb was a danger to humanity, but that it was very difficult problem to solve. it's taken more than twenty years to build LHC: if it takes several years to get it working properly, would that be surprising?