There are so many ways to improve the boarding of an aircraft (and like reverserbucket I am astonished as to how someone was able to apparently pass through an electronic boarding pass scanner and doc (passport) cross-reference inspection at the gate by Menzies staff)..
However as ever it comes down to cost and willingness of the staff to deal firmly with passengers. Most of the problems would go away if hold luggage was included in the ticket price. The worry (and it is a genuine worry for some people) that they will not be able to find a place for their large cabin bag makes them crush up by the gate. It is exacerbated in some flights by having large numbers of children (especially those under two) and parents who are not frequent flyers.
Alternatively, if you pay for luggage there is a guarantee that luggage is waiting on the carousel within say 15 minutes of the plane arriving on stand and stopping the engines. At least that way, you can be certain of being out of the airport within a reasonable time.