I wonder whether a lot of it is the Psychology of it all.
In Kings Cross, Glasgow, Paddington, you FEEL as if you are beside the trains. There is a gateline, there is your train.
In Euston there is just so much clutter with all the catering in the wrong places, that in the main concourse you FEEL as if you are some distance from for example Coach A of an 11 coach Pendolino on Platform 1.
In reality of course, when there is a late inbound for the 1830 they would be much better to put a "Expected departure 1845" on the boards and audio announcement, as you simply won't turn around that train in 0 minutes, and leaving it non specified creates the sprinter mentality.
If however the station were decluttered with much more public space, much more visibility of the trains, I wonder whether a natural improvement would come about from "of course we aren't going to miss it, it isn't even here yet". I also wonder whether the ramps could be better designed to, a la Easyjet etc, have queueing lanes so that you could announce the platform for the 1830 before the delayed arrival, but still have a decent egress path for inbound pax. Then for example logically you would board coaches A-C,D-F,G-K,No Reservations sequentially so you could regulate the flow along the platform and load those with the furthest to go first. As you generally do an aircraft.
Unfortunately I think if the cleaners were to face boarding passengers, their work would be considerably slowed, and sadly I think the great British public is frankly not considerate enough to give this time before hoarding on, so there is probably some need for a short "no boarding" window between arrival and boarding beginning.
So much of all of this is behavioural, and whilst there is still the odd passenger who gets to the front of the easyjet queue and looks as if no one has ever asked them for a boarding card before, most do "know the script" and get used to the ways of boarding, and can see its logic, whether it be to their advantage that day or not.
The reason so many good thinking people push back on the Euston situation is because they perceive it to be entirely bonkers. And they'd be right.