To reinforce a couple of points from above.
They must have a legal responsibility, surely, to ensure that the passenger standard weights take into account the probable increased amount of cabin luggage..
And I agree that it will be the poor cabin crew that will bear the brunt of this policy in dealing with f-wit passengers who try to sneak as much on as possible. Airline ground staff hate saying 'no' to passengers. Not entirely their fault as much pressure is brought to bear on them from higher levels to avoid pissing off passengers.
I take it the norm would be to audit on an occasional basis the actual weights of passengers and their carry on baggage to ensure that the standard weights used by an airline reflect reality. Will they introduce a special audit with such a major change in policy?
CASA would be remiss not to demand this of them as part of their monitoring responsibilities.