Reason I ask is not everyone will have carryon, and not everyone's carryon will be right at the 7kg limit,
KRviator, you have obviously never travelled with EK. How shall I put this? The... "95 percentile" EK passenger is pathologically incapable of travelling with less than 7kgs of hand baggage - and would (does??) carry 70kgs of hand baggage if he/she could lift it. (Granted, not on EK, but on another carrier serving the Middle East), I have seen a long succession of (admittedly tiny) Filipinas carrying backpacks so heavy that it took two others to lift each bag onto each one's back - and then, most could carry it only by leaning forward and when still, resting it on a rubbish bin.
Go on a Lagos flight and you'll find a whole new description for the term "Big Momma" - and the size of the hand baggage they
all carry is almost beyond belief.
Seriously though, what you suggested is pretty close to the industry standard. A standard passenger weight and a standard hand baggage allowance in theory allows for the percentage who will be above standard being offset by the hopefully equal percentage who will be below it. Likewise with the hand baggage. However, as has been suggested above, from daily observation, in many areas of the world, it may be time a new standard weight for both people and what they carry to be accepted and adopted.
Again seriously, any form of 'real time' weighing would be a commercial nightmare, which is why it hasn't been adopted. (Can you imagine the fights between captains and load control?) Boeing, and I'm sure Airbus, allow a generous 'bugger factor' in their performance calculations. I've (purposely) done a takeoff in the sim. in a 777-200 forty tonnes over MTOW with an engine cut (severe damage and separation) at V1 +1. It was curvature of the earth stuff, but we did climb away, if at a very low rate of climb, and managed a full cleanup.