But that works for as long as people enter through the side doors and have a reserved seat. In case of selling tickets without reserved seats people may have to stand. Also if you are going to attach / detach carts from the back or front of the train (which is more likely a scenario with unreserved seats) you'll get a lot of traffic inside the train in opposite directions anyway.
Again, where is this country where high speed trains sell standing space? I have never seen it of heard of anything like it. In China and in Spain riding the high speed train is more like an airport experience. You need a reserved seat just to get through security and there is no such thing as 'tickets without reserved seats".
And, of course, if this were a problem, all you need to do is not sell standing space and the problem goes away. But since they do not do this anyway the problem does not exist.
Again, where have you seen passengers with standing tickets on high speed trains? Do they allow standing passengers on airplanes as well?
I have not seen standing tickets sold for long haul destinations in Spain pretty much since I have memory. Maybe short haul but definitely not long haul. And in China I have also been told no tickets were available for long haul trains. I would have stood all night if necessary but was told not possible. Maybe it is different on other trains with shorter itineraries and daytime routes.
I am baffled by the notion that is it difficult or impossible for passengers to move up and down the aisles because it is done all the time. People move when they are getting on or off or getting ready to get off even if the train is still moving, or they want to go to the lavatory, or they are bored and want to stretch their legs. I have never seen anyone who could not move about the train.
I suppose you could make the case that someone on a wheelchair or other special needs might have difficulty moving about the aisles but that is just as true today whether the last car is going to be disengaged or not.
Maybe I am missing something but I just do not see the problem.