I can't say I agree with your logic here. We are not talking about any specific backup or anything like that. We are talking about the design of a file format that is going to be used globally.

Business running a lottery has to calculate the odds of anyone winning, not just the odds of a single person winning. Same, a designer of a file format has to consider chances for all users. What percent of users will be affected by any design decision.

For example, what if you would offer a guarantee that 32 bit hash will protect you from corruption, and compensate generously anyone who would get this type of corruption; how would you calculate probability then?