I don't believe OP has published anything about what they actually paid. But looking at October 2015.
BNE-JFK return can be had for $8600 in J, and $2100 in Y (looking at cheapest fares)
So the difference is $6500 (Obviously different dates will give different results - but I suspect this is ballpark)
Now the OPs parents were downgraded for 13hrs out of a 36hr journey (one leg out of four) (ie 36% of total journey)
About $2350 applying this percentage to the fare difference.
So in this respect I would view the overall QF compensation as fair, albeit certainly not overgenerous.
--
I agree the downgraded fare table itself (1200/1250) is pretty poor, but I believe in all these situations that a payment at the airport is generally made, so in my mind this needs to be included as part of the overall package (and is potentially reflected in the table) (presumably QF also has rules about airport payments)
Where I think QF has let themselves down in this situation is:
a. Seemingly the agent behaviour at check-in
b. The communication with the OPs parents - seemingly not enough, and simple things like telling them how much the refund would be, and proactively quickening the refund process via Flight Centre, were not done
c. Not fully explaining why these pax were selected for downgrade (per OPs post they had checked in early and were SG members). RedRoo posted a generic statement but it didn't seem to answer the question
While some on this board also think the number of thread views should make QF more generous, I actually think it has the opposite effect. I suspect they are being very cautious here not to set a precedent.