Property is not fluid and doesn't change hands very often. Sure, in a large enough market lots of activity happens every day, but any given property is only involved in a transaction once a decade. You can have very old backups and still capture the state of the market to 99%+. Any recent activity will have brokers, buyers, and sellers, who all have current copies of their activity.

Also, this sort of event should result in some hackers being found and jailed for life as well as their families being bankrupted permanently. Or, if they are being protected by their government, this should be considered an act of war and an appropriate military response should be delivered.