Good luck trying to get 23kg in a suitably sized carry on bag. Of course, people like to take on all manner of things as "carry on" these days - hence the need for tagging at the gate. Hand baggage only fares have made the problem much worse in recent years.
As for the complaints about self service machines, £65 quid, and "common sense": the common sense and obvious thing to do is to put the heaviest bag on first as indeed the instructions tell you.