The issue is not that you have an atomic copy of the state at a certain point in time, which in a bad database couldn't be loaded. The problem is that when you

The issue is not that you have an atomic copy of the state at a certain point in time, which in a bad database couldn't be loaded. The problem is that when you make a full copy of a file you are copying linearly and if it's a big file a chunk could change while you're reading it (or multiple related chunks which you are in the middle of copying) -- then you have a very inconsistent state which a database can't recover from no matter if their updates are atomically written. The database could do some magic with unlinking, but that would cause massive performance problems.