Of course, all airlines could fix the problem with carry-on luggage very simply by installing overhead bins with separators to create a individual compartments of the exact size of their maximum permissible carry-on bag. Label each compartment with a seat number - and presto.
It would at least stop a lot of the cheating that goes on(In the states I noticed a lot of the pax taking on 2 or 3 pieces of handbaggage!).