What about if this passenger gets on the plane with something illegal in their pocket, (e.g. drugs), and puts them in someone else's (distinctive, bright red) bag whist it's up in the overhead bin? They then text their friend at the destination airport and ask them to look out for the passenger coming through arrivals with the bright red bag as the drugs are in there.
On the hypothesis that they were going to do this, why would it make any difference if they stayed on the aircraft or got off?
We were on the runway in Manchester (A319) awaiting the doors being closed
That would be a bigger concern! You mean the apron. The runway is the long strip of concrete that is used for take off and landing.