I would bet if the airline made an announcement, before the flight, that anyone who tries to board out of their zone is booted off the flight, everyone would comply. There are the 95% who play by the rules and the 5% who have to be forced to play by the rules. That being said, the airlines are at fault for not strictly monitoring carry on luggage, both amount of luggage and size. The aircraft are actually manufactured with overhead bins to accommodate every passenger with the appropriate carry on luggage. If the bins are overloaded the airline is to blame.