Looking at the current Building state, the only choices left for swapping Blocks without causing a Collapse are the two on the third Level. Once one of those is swapped with an Empty Block on the topmost Level, there will be no Blocks that can be swapped without causing a Collapse. That moment seems like a good point to say that the Building is Finished and allow a Move On without having to wait for 48 hours, though we still need the 48-hour period in case no one is brave or active enough to get us to that point.