To c&p a comment I made on LJ:
I keep getting this feeling that the crack doesn't actually DESTROY anything, but that it rather just erases all memory of something. All the events--Cyberking, Dalek Invasion, etc.--still happened, but the MEMORY of it has been erased from human history.
Interesting theory. I really enjoyed the series and am looking forward to series how the arc pans out. I think the series will be even better when watched after the final, there's probably stuff we're not noticing.