I am still amazed that in this day and age of technology we are still using 'standard' weights for pax.
How hard would it be to put a low-profile weighing load-cell in front of the check-in counters., you will then have the pax+luggage+carryon weight.
I mean, at the flaming poxy 'self check-in' kiosks there already is a weigher to weigh the hold baggage, so why not expand the footprint a bit to include the baggage+pax+carryon ?
Sure., there will be technical issues such as people clustering at the counter or self-checkin area, but a bit of 'could you please step behind/off that line' and some basic sanity-checking of the total measured weight would solve that.
If the airlines measured the real pax+baggage+carryon weigh they could then maximise their freight load, but only after taking in to account the captains view of how much fuel to take for the trip + Wx + probable delays.