Again - goes back to what the airlines identify is the problem, is it a
volume problem or is it a
weight problem (everyone taking 6 compact house bricks or everyone taking a large box onboard full of feathers?). The first could be described as a weight problem but the second is more of a volumetric limit problem. Enforcement takes time and resources so makes sense to get some sort of self enforcement or easily enforceable guidelines that are simple and undeniable.
Interesting discussion about the weight and balance and assumptions made about pax and bag weight, but I think there is enough operational knowledge and such a large safety margin built into aircraft performance charts and loading data that its not someones laptop that is going to make an aircraft over-run a take-off.
The engineering of the actual overhead bins is worth discussing further, and whether the overhead bins "cube out" on capacity
before they get close to their engineered weight limits? Or not.
Some economic stories in here as well with ancillary revenue and the perceived value of time driving the decision making process of how much is checked in and how much is attempted to be brought on board as carry-on. If the ancillary revenue has the
unintended consequence of getting more luggage on board as carry-on, with the subsequent delays/gate check shennanigans and overhead locker rage that we see in the US, then logically its just a matter of the different accountants measuring and assessing the cost of delays vs the often illusionary benefits of ancillary revenue. I.e. there's no point having the best ancillary revenue flow if none of your flights stay on schedule, your fleet burns more fuel and you end up with aircraft and crew late or out of position or curfewed out is there?
Although expensive to install - maybe the foot lockers (see link) will mean that airlines will be able to "sell" the existing overhead locker space as another revenue stream? And if so - will they be selling it by weight or by volume? Will it be cheaper or more expensive than checked baggage?
https://edition.cnn.com/travel/article/airbus-fly-your-ideas-competition-foot-locker/index.html
I can see scanning technology not far away at all that could assess pax size weight and volume (and their carry on) being rolled out in the airbridges or at the gate so that the software will know exactly the weight and dimensions of every pax and their bags as they go onto the aircraft. The fuel and time savings of that could make this technology extremely attractive to airlines. I wouldn't be surprised if a technological solution is attempted in the future.