The two years validity only apples to the individual exams - all exams must be completed within two years of each other.
Once you have passed them all you are awarded an ATPL exam credit. Currently this does not expire. You could hold it for 30 years without doing the flight test. CASA has said this may change but at the moment the exam credit (all seven exams passed within two years) lasts indefinitely.
So you may as well complete them if you think you may need an ATPL at some point in the future.