Halfway down a single 156 isn't that far, about 22 metres or so from the front door, and if you were spritely getting off aswell then the Guard might not be leaving enough time between opening and closing the doors. That said, if it had previously happened to the 'gentleman' recently, I'd expect him to have been a bit quicker off the mark.
It should really be accounted for in the timetable, but that means longer journey times that no-one wants. It's a bit like the minimum suggested interchange times at stations, there will always be people saying it's too long and others saying it's not long enough.
It seems a reasonable suggestion, but how do you judge who needs help and who doesn't? Sure some are obvious, but some aren't, and there are many people out there, some of whom who really would benefit from a bit of assistance, who would react badly to even the suggestion that they might need that assistance.
Sadly this is not restricted to the older generation. Many people leave things to the very last second without planning ahead. In one case I had, a passenger, with family and luggage, turned up for a train an hour after the last connection for their ferry, the last of the day apparently.
30 seconds is good if there is room in the timetable, sadly in some areas this is only provided at major stations, with smaller stations having arrival and departure timed either 30 seconds apart or at the same time.