Of course, your odds of winning Tattslotto are better than all the IP's burning down.I think this scenario would certainly mean a visit from the Arson and Fraud Squad ....
The chances of them all being vacant are fairly slim as well, unless you've only got high-end, more boutique properties, and/or in bad locations.
Property selection - which amazingly falls under the heading of Due Diligence - will hedge against that quite well.
We had 3 out 5 vacant at the same time once, but it was short term, and in the event of one being harder to rent, you simply drop the rent a bit below the suburb average for that type of property and it will rent.
Not ideal, but it is not a horrendous situation as some might think.