Are you confusing bumping due to overselling with cancellation of an entire flight? If flights happen to be running at 85% load factors (not uncommon) that means you have 45 empty seats on a 300 seat plane. I really doubt that QF are going to drip feed their cancelled passengers 45 at a time over the next 5-6 days onto the same flight, especially when they are operating a JSA codeshare on the same route with a strategic partner. They will find solutions.
Aren't these just normal operational factors in running any airline, not specific reasons to not go ahead with a PER-LHR flight (there may well be other good reasons for that) If an aircraft goes unserviceable in PVG, JNB or SCL and the flight is cancelled, what happens then? Maybe they shouldn't fly to JNB or SCL because of the consequences if the flight gets cancelled and passengers might have to wait 24-48 hrs for the next flight.