Various articles have said (and maybe someone here also said it earlier---) the airplane crew could have left the deceased person in place, and moved the other two passengers (the Australian couple) to Business Class.
There WERE available seats in Business Class.
But as many people around the world have commented, GOD FORBID that people who paid for Economy tickets would be moved to Business Class.
Especially for a snooty airline such as Qatar, it's practically against a law of nature.