If a CC2 building collapses, the expectation is that in only about 3% of the cases this leads to someone dying. I don't know the complete reasoning, but can imagine some factors of why this number is far below 100%: a building is not always in use, there are often warning signs with time to escape, and collapse can be localized (not the whole building).
Makes sense, so this is confounded by the number of people in the building.
30% for a CC3 seemed high to me initially (hence wondering if 0.3 really meant 0.3%). But since it actually means "in 30% of structural failures in CC3 buildings (e.g. a stadium), at least one person dies", it make much more sense because there are probably lots of people in the stadium.
I agree, it would likely have extremely diminishing returns in terms of lives saved to have more stringent safety requirements. Needs to be a balance after all.