There are rules that are sensible and practical, and there are those which are not.
While the rules are to be observed, it still does not mean it is sensible or practical.
At the rate QF damage people's bags at certain airports (= SYD, in particular) I am not surprised people would rather not check bags if they can help it. Waiting for 30 minutes for bags is also quite galling when you only have a small bag that would have fit OK and you could lift into the overhead locker and close the locker yourself, but for the 7 kg limit.
Then there is a risk of loss or delay.
How much does an average modern-day wheeled bags weigh nowadays? 2-3 kg? How much does an average laptop weigh, including the charger? 2 kg? That's already up to 5 kg then, before you put anything else in there.
Is that a sensible and practical limit the an airline should have in the modern world? Perhaps not.
Do we have a far higher injury rate in Europe due to overhead baggage because of most carriers having a higher weight limit than 7 kg (with causation established, preferably)? That would be an interesting question.
If a full-service carrier with equivalent stability, reliability, service and network was operating in competition and start offering 15 kg limit, let's say, then things might change very rapidly.
Is there any incentive for QF to change things as things stand? Probably not. Not until Virgin starts going the same way as the likes of easyjet and BA in Europe and essentially remove the weight limit on the proviso that passenger MUST store it themselves. Whether that would pass CASA's requirements or the union etc. would be another matter.