Members of the public are well used to automatic doors in many walks of life. Trains are just one of many such cases they encounter. Lifts in buildings (and indeed in railway stations, getting you to the platform) are familiar to all, and the manner in which their doors retract again if you put your hand between the door leaves is widely known. I bet the RAIB office has lifts just like this. Nobody would expect the lift to just start off and chop your hand off, and no lift manufacturer could possible get away with such a product.
So we have to ask why are trains different.
IMHO, trains and lifts are very different due to the difference in the numbers using them.
Only a small minority of lifts have a capacity of more than a dozen people and most of those are seldom used by more than a dozen.
Trains by contrast routinely carry hundreds.
Placing a hand between closing lift doors should cause the doors to open again, this delays everyone else in the lift and is irritating to those persons. This is considered acceptable in view of the small numbers involved.
Delay of say 15 seconds to say 6 people is a total of one and a half delay minutes.
Now consider a rush hour train with 900 passengers. If the doors re-opened when obstructed, a 15 second delay is a total of 225 delay minutes.
In practice it would be far worse, on a busy train several doors might be held open, and remember that each re opening would probably attract another latecomer costing another half minute at least whilst they try and board.
I doubt that an effective rush hour service could be run if train doors automatically re-opened when obstructed.
